When my AC turns on the fan doesn't always turn on when it is set to auto. Sometimes it does sometimes it doesn't. I can set it to on and the same thing happens. If it turns on It will stay on and turn back off with the rest of the unit. Right now I have to run the fan all the time so the unit doesn't turn on without the fan and keep the compressor running because it isn't circulating and cooling the house. What causes this?

There could be a fan relay in the furnace/air handler that is intermittent causing this problem.

You did not give much info on your equipment. It could also be a relay on he circuit board if it is a newer piece of equipment.

If there is a delay when the blower starts, it's probably a time delay relay or sequencer some use that's failing intermittently. Replace it.

If it's a digital t-stat, that may also be the problem. I don't particularly care for the digitals. One more problem in the equation. They're an unpredictable animal.

Having just experienced a similar problem after 2 different A/C techs told me there was nothing wrong with my unit. I made sure the t-stat bypassed the fan relay at the heating unit. Next I bypassed the time delay switch and ran the wires directly to the contactor. Problem solved. Since the digital t-stat has a built in time delay, you can do this without hurting anything. 2 time delay switches are redundant anyway. Good luck with it. Sometimes common sense is better than a certificate.