I have a Rinnia hot water system that keeps going cold in the master shower.  In the upstairs shower, it is constant.  But in the master, I get what I think is called the "cold water sandwich."  The only difference between the two showers is the head, which in the master is a Delta low flow that has the massage option.  Is the low flow my problem?  I have to turn the lever (single) all the way hot, and then ease off of it about 1/4 of a turn.  If I go farther, it gets cold.

At the moment the problem presents itself, conduct an isolation test, by leaving that shower running and checking hot temp at the bathroom sink.
If water is very hot at sink, then maybe you might consider replacing the cartridge (which likely contains a thermostatic element) within your shower valve.

If the bathroom sink hot water temp is poor at the same time as the shower,
Consider if your hot water pipe conveying to the bathroom may be getting cold water cross connecting into the hot pipe casing diluted temp. This problem is common if the homes toilets have a failed mixing valve to feed the toilet warm water during humid months. Most homes however don't have toilet mixing valves.

Definitely sounds like a flow issue

I will do that...how hard is it to replace a cartridge?  The level is already a little loose, even though I have cranked on the allen screw that holds it in place.

I have the same problem.  I can't remember the brand name, but trying to take a cooler shower in the summer months, a few minutes in and lose all hot water.  
There's a hand sprayer, too, so I've been running the hand wand and shower head, and haven't had the problem again.

If you've performed the isolation test to prove the problem is only the tub/shower valve... And problem not occurring at bathroom sink at same time...
You may have a hot limit stop which you may adjust in order to permit the handle to turn into the hot some more.

If that doesn't help, then yes, replace the cartridge. Most brands call the part a cartridge, but there are exceptions such as Symmons who call it a spindle.
If you decide to replace the cartridge, before you take the valve apart, try to identify the valve manufacturer and specific model number.
The desktop version of delta or Kohler websites have a nice way to assist you identify your model number.

So when I need parts, sometimes I can't find the one or two pieces in stock today. In that case I can usually purchase a complete rough valve with cartridge and they're not expensive and are usually in stock at my wholesalers. More parts before disassembly is better.

Delta valve may have about 6 different cartridge designs. If it's beyond 10 years old I'd usually replace the bonnet nut which retains the cartridge in the valve body, after cautiously cutting the old bonnet nut in the same type of method you would use to drill and split a seized nut off of a bolt stud. The reason I prepare for that with a delta valve is that the bonnet nut often seizes onto the valve body when one of two pitfalls may happen: 1.) wrench tool eggs the nut & valve body out of round.
2.) seized nut torques half of a two piece valve body to shear apart on a Delta valve body older than 1999. Those older valves have three 5/16" copper tubes brazed between the brass Body halves, and must be treated cautiously.

Some valve brands such as Kohler have a pressure balance spool located deeper inside the valve body, behind the cartridge.
For the Kohler and Symmons valves in particular, I find it most practical to get a complete rough valve and gut it for parts.

before I'd change the cartridge I would check in the shower head for flow resrictors

It sounds like you are not meeting the minimum flow requirement for the tankless water heater.  Look into the Navien brand of tankless with a buffer tank.  They don't have a minimum flow requirement.

It's a delta handle, can't remember the exact model, but it is only 7 years old.  The handle jiggles on the connection as well, so I will probably end up just replacing the cartridge.  Thanks everyone.

Sometimes a mixing valve lets hot bleed over or cold bleed to hot. Maybe check the shower valve.

low flow shower heads and tankless water heaters are the devil.

I'd try swapping the shower heads between bathrooms and see if the problem migrates with the heads

Thanks for that recommendation.  The problem head/handle is a Delta Monitor.

I replaced the cartridge today, and while I was at it I took off the head.  The old cartridge was really dirty inside, and the screen on the head had a bunch of junk in it.  So I don't know if the new cartridge was the trick, or cleaning the head.  Regardless, it is fixed.  Thanks everyone for the help.

the important part is that it works!

Thanks for the update