Thermostat losing power *after* AC completes its cycle

A little background: We have an Ecobee thermostat we installed ourselves 4 years ago. We had an issue with not being able to access the C wire in the wall so we used the PEK. That worked until recently when the thermostat started randomly losing power. We called a tech guy out who was able to access the C wire and get it wired properly without the PEK. That was about a month ago.

Thermostat seemed to be working fine/not losing power until last week when we noticed it was off after it reached the desired temperature on the AC. I started watching it and the same thing happens: AC turns on, runs until it reaches the desired temp, turns off, then the thermostat loses power within 5-10 minutes afterwards. It doesn't matter how long the AC runs. I've watched it turn off after it only ran for 10 minutes and another time after running for 3 hours. The power eventually comes back on on its own, but it sometimes takes several hours.

I shop vacuumed the drain line in case it was the float switch but it's still doing this. Any idea why this is happening or anything else I can check before calling the tech back out?