what's wrong with my air conditioner?

While it is within the realm of possibilities. No one here can tell you if it is in fact the issue. Motors can work then not when nearing the end of their lives, they hubs or fans can also come loose and give the appearance of working but don't actually pull enough air through. There are other little things like that but can only be verified by being there. You also want to look at the history of the unit, did you just turn it on a couple of days ago, humidity at the time, and so on. I'd get a second opinion and take it from there.

The condenser fan motor must run to cool the condenser coils and the compressor. Without one running, the compressor will kick off on internal overload. Then, the air will not be cold. The compressor cools off and restarts and you get cold air again.

The price is fair, too.

If the unit was running properly while the tech was onsite he cant diagnose whats wrong. He could tell if it was running at high amps, which could cause the motor to overheat and shut off. But there could be other problems as well. I'd wait till it shuts down again and get someone back out. $300 for a condenser fan motor replacement is not out of the ordinary, its a suitable price

Call another company and get a second opinion. You should also check with the better business bureau to find out if the company your talking to has any unresolved complaints. If so, steer clear.

Sounds like you're low on freon . Would definitely get another opinion

Call another tech and have them check it out.