First thing to do is get a compression check on the engine performed. If low in any cylinder, remove the compressor cooling lines from the engine, and plug the holes. Recheck. If the readings are good, the problem is the compressor. If the check still presents the same problem(s), it is probably time to remove the head(s) for either a blown gasket, cracked liner, etc. Have you checked for engine blowby? With the engine idling, remove the oil fill cap and is there positive pressure, or slightly negative? Positive pressure is indicative of internal problems brewing in the rings, and or head gaskets also. I'd be willing to be you are looking at head gaskets. Don't do just one however cause the other will follow suit if you only replace the existing bad one, (if the case). Not a big or expensive job if you can perform the labor yourself but things are heavy. Rob