Hi, I've got a Heat Pump system in my attic that seems to be cutting off unexpectedly. I've got two different digital thermostats (powered via the C-wire) and they both lose power and cut off unexpectedly, apparently after the system's been running for a while. I checked the voltage from R to C and I get 26VAC, but I don't know how precise my multimeter is at that range.

I can get them to turn back on if I kill the HVAC at the breaker for a little while (somewhere between 10 and 30 minutes), but then it goes out again a while later. I'm not certain on the exact time/workload it takes to make it shut down. First time it happened a couple days ago and I thought my fancy wifi thermostat died, so I replaced it with my old dumb non-programmable one. It happened to that one while preparing dinner yesterday, and again sometime between dinner and bedtime last night.

Any ideas of simple things I can try to get it working normally, or should I get an HVAC guy out here? Or would I need an electrician?

Sounds like it is kicking out on high head pressure, and has to be reset. Check filters, evap fan and evap coils and make sure they are clean.

YES, you should.

Going out on high or low pressure would not kill power to the tstat.

Sounds like the tstat is getting a voltage spike.

I remember another guy here with the same problem. If I remember right he had a line volt wire going to ground on the outdoor unit. Chassis ground is the C

This sounds like your 24v AC transformer is bad to me. The thermostat essentially operates independently of the unit itself, and the fact that you say it's dead means you are losing your 24v AC somewhere. That doesn't come from the unit itself. It would also explain why it's shutting down. 

I'd bet that the pan/coil drain is draining slowly and the overflow shutoff is wired in series with the R wire to the stat so it can shutdown the unit.

drain pan overflow safety and blower door are the most solid answers so far.

high head wouldnt cut the tstat out, the air handler and condensing unit are on separate circuits(well, i hope they are), and the high head would kill the compressor only and maybe the CFM as well depending how its wired.

alot of the times as well, if theres a circuit board supplying your 24v and controlling the air handler, that could be causing issues, but i find it highly unlikly because if that was the case, it would not have intermittent issues. it would just fail and not work. 9 out of 10 times.

transformers 99.99% of the time do not fail intermittently, its either working or not. but electrical can act fucky at times.  so dont rule that out just yet, put it last on your list.

intermittent issues like this are usually caused from a safety switch (door switch, overflow switch) or a loose wire wiggling due to a vibration.

goodluck, check the dumb stuff first, if you cant figure it out, call a tech. you dont want to blow up boards or fuck anything up.  tape the door switch as if the door is closed, and see if that fixes it, look at the drain line, check thermostat wiring in the airhandler for snug/tight connections (hit the service switch before you poke around)
another stupid thing to check is to see if the breaker in the panel for the air handler became loose, wiggle it around and push it in tighter, ive seen that a few times.

Agree

Ding,  ding.  Winner!

Find the end of the condensate line and using a wet rag to make a seal put a decent shop vac on the end.

It often clears very well.

Thanks for the tips. I'm HVAC dumb, so I might need some barney style. What's the safety blower door, pan/coil drain, and what might the overflow shutoff look like? I can try to post some pics tomorrow.

Your unit should be sitting in a big metal pan.  Its there just in case the unit is not draining out condensate properly.  If the pan fills up with water, there is a switch mounted on the side of it to kill power until it evaporates or drains out.  Once you look at it, it is obvious.

I got a new unit installed a few years back.  It was slightly off level and the pan filled up and did the same as you are experiencing.  I had to call them back out to level it properly.

It was the condensation drain pipe. It was quite clogged, probably from before we moved here (we've only been here a month). The drain pan wasn't filling with any water, but it was triggering the shut-off from an overflow sensor.

Yay! I was right.    While your at it you might as well go for gold and make sure the unit has a good filter and that the outdoor unit is nice and clean.