If someone is telling you that it will cost $350 for R12 run, don't walk, away from them and don't ever do business with them again. They are STEALING FROM YOU!!!

Follow engatworks advice and buy your own R12. Additionally, from what you've said so far there is not yet any evidence that you even need any refrigerant. Diagnose before you act.

The first thing you need to do is put gauges on the system and see what the pressures are.

Check your sensor wiring to make sure it is all in good shape. Your symptoms still sound like a slipping belt. Possibly the sensor is flaky or wiring to the sensor is flaky.

Look before you leap,