Air Compressor/Condensor Unit Operates Sporadically

I noticed the other day that my Nest thermostat was always showing the current temperature many degrees (5 or more) higher than the level I had set the AC to. After a little investigation, I found that the fan was on and blowing air, but the compressor outside was not. Since I had installed the Nest last fall, I assumed it was a wiring issue I had not encountered because I hadn't used the AC until very recently.

I spent an hour on the phone with Nest support and they believe everything is wired correctly, so yesterday I was going to call an HVAC repairman.
That morning however, I turned the AC back on (I turned had turned it off for the night after failing to fix anything with Nest support) and the compressor came on too. Throughout the day I noticed it both on and off when the AC was in cooling mode (set below the ambient temperature). To be sure it isn't a Nest issue, I reinstalled my old thermostat, and I've found that the compressor will come on and cool the house, but when it reaches the desired temperature it goes off and doesn't seem to come back on.

For example, the room temperature was 73, so I set the thermostat to 69. A little while later I noticed it had cooled to 69 (and the compressor had been on), but then it shut off and the room gradually warmed back to 71. From there I lowered the thermostat more to 66, but the compressor didn't come back on in the 20 minutes I waited, and the room continued to warm another degree. I shut the AC off completely, waited 5-10 minutes, and turned it back to cool -- the compressor came on.

Last night I left it set to 71, and it was 70 degrees when I got up.

So, am I crazy? Is something wrong here? I was under the impression that the compressor unit should be on whenever it is cooling the air to a temperature lower than the current inside temp. Is that not true? I don't want to spend 100 bucks to have someone come out to my house to tell me everything is operating as it should. Does anyone have an idea of what could cause this behavior?

Thank you guys so, so much in advance!

 

Sounds like the stat is ok. Problem is probably in the outside unit. Does the outside fan run when the compressor is running?

 

yes, the fan in the compressor is on when the compressor comes on.

 

You need to check 2 things: 1) Did the T-stat send 24V to the contactor ? 2) if it did, was the contactor pulled in (turned on) ?