I had this issue with my previous old unit and this one that was installed approx 3 years ago.

This is a heatpump system and I have the issue in heat or cool mode and have replaced the thermostat.

Randomly the outside unit will not power on (or shuts off while running, not sure which). I discover the issue because the fan is running without it getting any warmer (in winter) or cooler (in summer). I have 2 double breakers for the system and neither will be tripped. I can turn off the thermostat and both breakers for a couple mins, then turn them both back on and set the thermostat and it works again.

Some research I have done may indicate I need a hard start capacitor. Or, that the the system may be overheating due to not enough airflow due to closed off vents etc.

I do have a couple vents closed off but seem to have a lot of airflow through the many other vents I have. Filter is changed regularly. One other thing of note is that my house is about 40 years old and on a slab. One company that I got a quote for when replacing the unit mentioned that they may need to run new tubing through the attic as the tubing going through the slab is too small for a modern system. The company that ended up replacing it didn't think it was an issue but did have to use a reducer on the lines.

So, I am thinking of replacing the breakers for good measure and then consider a hard start cap (they are cheap enough). Other than that, any suggestions on what the cause is or how to try to track down the cause?

Thanks