AC will activate fine in the morning but not in the afternoon or evening. Furnace fan will run but AC unit's fan and compressor will not. If left to run all day it's fine. Will run all day. Is only connected to a basic thermostat. Any ideas?

Is the thermostat programmable?

No it's not.

It's hard to say, what are the brands and model numbers for the system? Also, how handy are you? I can give you a few tips on some trouble shooting but your going to need a few tools, like a voltage meter.