blower control knob issue?

I just picked up a 1991 300GD and I was told by the P.O. that the AC didn't work. After playing with it i found that the freon was a tad low but the compressor kept cutting off when it shouldn't. I discovered that if i turned the round blower control knob off and back on that the compressor kicked back on. Then the compressor would quit working again a few min later, once again i turned the fan blower off and back on and the compresor started back up. I assume at this point that it's a contact in the fan control knob itself. Does this sound like a possible diagnosis? Is this a common problem?

Thanks,
Doc