I have a 1990 model pick up with a c problems. I have replaced the compressor, orifice tube and accumulator twice. I added pag oil to the compressor each time and I let it sit for over an hour and spent the compressor while adding the oil. I took it to a professional shop each time to have the system vacuumef and freon added. Each time the system cools but the compressor squeels/roars under low rpms but stops when under acceleration. as I come to a stop it starts to roar again. can someone tell me what my problem is?