The fan runs all the time but the compressor seems to short cycle on for 20 sec and then off for 30 or so.

The compressor is probably overheating and the safety is turning it off. Check the fan on the evaporator, not the inside fan and make sure you have freon at the proper charge.

I agree with ma0641 to check the condenser coil fan. He said evaporator coil but I think he means the condenser coil. The one underneath the bottom. I agree that the compressor is probably shutting off because of the overload switch. But it is likely that the overload switch is defective. If the compressor is truly over heating it would take more than 30 second to cool down. The overload switch is usually held against the side of the compressor with a clip. Remove it from the compressor and let it hang free and see what happens. If the compressor is truly over heating it will be very hot to the touch.
Don't see how you could check the refrigerant unless you have a set of gauges and then you would have to know what the pressures should be.

Yep, condenser. Another gray matter attack!