your compressor is probably starting to leak internally by the head gasket and/or the pistons. It is just a miniature engine just like your 5.0 liter and can develop the same problems after 20+ years. Luckily, it is pretty easy to overhaul the compressor with the correct gaskets and o-rings. Or you can just buy a rebuilt compressor.
> '86 928S 5-spd w/LSD
Actually, a good test to verify if the internal valves are leaking is to run
the compressor until the suction line gets very cold. At that point, turn off
the engine and immediately touch the suction line closest to the compressor to
see if it gets hot. Basically, a cracked or improperly seating hot gas discharge
valve will bleed hot gas back into the compressor and into the compressor
suction line. If this is the case, compressor needs to be repaired or replaced.
'91 S4 Horizon Blue/Cobalt
PCA-Rennsport Region Montreal