My upstairs ac unit won't startup when the temperature is above 82 and attic humidity is high. The thermostat seems to work OK. On cooler/dryer days/nights the unit starts-up fine. Could this be a blower run or startup capacitor issue? The control unit was replaced 2 yrs ago.

Blowers usually have run caps not start caps... could be a blower motor issue. Coule be a wiring or power issue... would need more info. Might want to call a tech out to check it too.

Appreciate your response. So you think it might be a wiring short or power interrupt? Unit is a ComfortMaker RPJII Model GUI050A012BIN. In the evening when T-stat timer calls for AC nothing happens, no inside blower, no outside blower. The next morning when temp/humidity is down same ac request, runs fine. In the meantime there were no power resets (OFF/ON) at the unit or at T-stat. Also, when the unit isn't work setting the blower from AUTO to ON at T-stat does nothing. When unit is working setting blower from AUTO to ON turns on blower as expected.

Yea I would have it looked at. If it is a 24 volt problem they can be tricky and hard to find.

Might try pummling wires from tstat. Make yourself a diagram. Cut the wires back and make fresh connections. This might help. Might not.

Found it. Not jiggling wires or bad connection! Clue, blower will not turn on from t-stat in either ac or heat mode when attic unit temp goes above mid-80's. Replaced blower motor run capacitor and all is well. $5.20 not a $300+ tech service call!