Helping a friend with a 71 Corvette A/C system. Problem has us really scratching our heads. Pro6ten compressor, new parallel condensor, POA eliminator, New expansion valve, 134 refrigerant. System starts and operates normally , after a few minutes the compressor starts to really load up causing the belt to slip bad. Turn off the compressor and restart works ok for a while then starts to slip again. Low side pressure is operating in the 25 to 40 psi range of the POA eliminator switch. High pressures are less than 300 psi. Leaning toward to a bad compressor since we have replaced everything else. Any suggestions?