Air Condtioning problem

I have a 1992 Cadillac Brougham 5.7 liter engine with a R-12 refrigerant ac system, 53,000 miles. The system is fully charged but at idle the compressor will disengage and not start again unless A) I high rev the engine B) tap on the canister molded onto the high pressure hose where the connection for the manifold gauge hose would attach. Am I dealing with a contaminated system? Any advice appreciated.