Sounds like the relay is telling the compressor to cut out. Could be the rpm portion of the compressor. Its part of the comprssor but you have to pull it and rebuild to get to it. They get dirty and cause intermittent faults. You could try brake cleaner and spray it down to get all the crud off, not sure if the old seals would like that tho.

Also freon charging entails you reving the engine to 2000 rpms to draw in refrigerant enough to hold a system charge. Compressor must be engaged full time. AMbiant pressure with car off should peg in the red. When you start car and compressor engages should be like 50 psi low side, when you hit 2k rpms it should be about 30-35.