Compressor skipping at idle

[zenuff]

Well I have my ac system working now on the 79 240D. I replaced the expansion valve and filter dryer. Charged with r420a and getting some good cold air at the vents.
The problem is when I come to a stop light and the engine is idling the compressor starts locking up. Like it's under so much strain to push the fron out it starts skipping and making the belt squeel. If I speed up the engine it's fine. Tightened the belt and it's still the same. I watched and it will stall and spin and stall over and over until I accellerate the engine.

I think I may need to get a compressor and redo this thing to get it done right.

Btw what is a proper pressure reading? I have 40 low side, 150 high side.
Does this seem normal if I'm running it at pressure that are supposed to be close to r12?