I would suggest replacing the compressor and dryer. Have them flush it, pull a good vacuum, add the necessary amount of oil and then top it off with r12 refrigerant. Some other members may come along with some more suggestions but this should get you started. You can get a compressor at AutoZone, Advance or places like that (just give them you car make/model). Only purchase a new compressor - don't try the rebuilt route.
Try to stick with r12 or go back to it if you are now running r134. You will need to make sure you have eliminated all the leaks in the system too.