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Lots of good suggestions so far op.

If you can check both legs for voltage feeding into your main breaker and coming out are they the same? Could be as simple as a worn out main. Going through temperature cycles could be what's causing it to be sporadic presenting problems.

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At worst, a loose neutral would get you 240 volts instead of the expected 120. Which will certainly damage attached devices designed for 120, but doesn't affect the wiring at all. His wiring is fine.

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Ever since I put LED lights into our house almost 2 years ago, they would randomly flicker.  It only happened a handful of times a week and only lasts a few seconds.  I just chalked it up to LEDs being LEDs.

Within the last couple of weeks my UPS battery backup in my office started beeping as soon as the lights flicker.  My printer that is not on the UPS reboots itself.  The backup is on a different circuit than any of the LEDs.  We have the same LED canned lights on about 3 other circuits and they all flicker for the same amount of time.

My UPS has a utility on Windows that monitors power outages, under voltages, and over voltages.  Even though it beeps it does not record anything in any of those categories.

I'm at a loss to figure this out.

I also checked and my HVAC is not coming on when this happens.  Neither is my fridge or air compressor.  In fact any of those can come on and the lights don't react.  The only other appliance I haven't been able to track is the small freezer we keep in the garage.  Nothing else uses a lot of electricity in our house.  So I'm thinking if it's not the freezer then it has to be a voltage drop from the transformer?  

UPDATE:  Still no luck

Tightening the neutral bar and a few neutrals and grounds in my breaker panel didn't seem to change it.  Still did it Mon, Tue, and Wed.

I called the power company out and they came out yesterday.  He said the neutral at my main was just a little bit loose, but everything else looked good.  He did re-tighten it and requested a recording meter from corp, but didn't know if he'd be able to get one of them.  

So I needed a better way to track them.  My CyberPanel UPS software was just the personal edition and wasn't catching it all nor did it keep a log.  Looked into the Business edition and it would log every event. So I installed it.

Turns out it's happening a lot more than I thought.  

I thought it might be the freezer out in the garage, but it's unplugged.  Only other thing is the hot water heater.  You can't tell when it comes on.  So I'm going to turn it off today and see if that fixes it.

Any other ideas?

https://www.AR15.Com/media/mediaFiles/167167/2017-03-23-06-14-00-PowerPanel-Business-Edition----Agent--Logs---Event-Logs--171972.JPG

UPDATE 2:

It seems to be happening at around the same time every day.  Now that I have the UPS logging it I'm starting to see a pattern. Tuesday and Wednesday weren't completely logged, so I was relying on seeing it myself and probably didn't catch them all. The automated logging started at around 7:15 PM Wednesday.  If the pattern persists it should happen again tonight around 10:30.

Also, I unhooked the water heater at about 11:45 AM Thursday morning to see if that was the culprit but it's not. I've isolated every single appliance at this point, including the TP Link Powerline LAN adapters. I also don't have anything on a schedule like this. My outside lights come on at dusk and off at dawn. So that's not it.

Monday
10:38 PM

Tuesday
7:33 PM

Wednesday
11:19 AM
7:25 PM
10:23 PM

Thursday
12:34 AM
3:27 AM
3:34 AM
11:24 AM
7:31 PM

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Can you post a pic of the transformer feeding your house ?

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I'm not nearly as knowledgeable as others here but a sales guy at Menards once told me I needed special dimmer switches for LED. Don't know if it's true. I mention this because you said it started after you put in LED bulbs.

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That alum wire to the transformer is likely not in conduit, probably direct buried.  Conduit only goes 18" underground.   If the wire got damaged during install the problems could just now be showing up.

Hopefully they found the issue in the meter today.

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Have you ruled out the possibility of a poltergeist ?

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Have you asked your neighbors (on the same transformer) if they get the same problem?

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loose neutral?

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Yep

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I'm just curious, but have any of your neighbors that share the same transformer as you had any issues?

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Digital MM's are not good for catching those fast dips/swings in voltage.  It would probably show up better on a cheap analog meter to be honest.

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He should be able to set the Min/Max to Min and be more likely to catch a drop.  Just watching the display, you are dealing with whatever integration and averaging the meter is using and a blip isn't as easy to see.

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did i read it right...they are on a dimmer switch??? If that is the case, that is your prob. dimmers are rated for wattage, some LED lights are dimmable but act weird as F and do goofy shit.
ended up being my issue. My girl bought a cheap ace hardware dimmer...I changed it out to a more expensive dimmer for LED and problem was solved

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did i read it right...they are on a dimmer switch??? If that is the case, that is your prob. dimmers are rated for wattage, some LED lights are dimmable but act weird as F and do goofy shit.
ended up being my issue. My girl bought a cheap ace hardware dimmer...I changed it out to a more expensive dimmer for LED and problem was solved

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Hmm what else would be periodic?..  sump or ejector pumpon a cycle timer?  Hmm you don't have a hot tub do you?