IF the belt doesn't move, compressor may be seizing up when it gets hot? Converting to R-134 would be a good idea, becuase R-12 is helluva expesive and is rare nowadays. It's always good to replace the orifice tube, condenser, reciever/drier, and you may need a compressor. Take it to a shop and get a A/c check so your not just guessing on shit here.