Your problem with brief periods of heat is not likely caused by refrigeration problems. It is more likely a heater valve problem or compressor control problem.
The evaporator job with 5 vacuum servos (at $20-35 each)and retrofitting is around $1500. Putting it back in R12 (Freon) is about the same (higher refrigerant costs versus retrofit procedures). I don't recommend retrofitting old nippondenso compressors especially those prior to 1990 (smaller displacement - lower capacity).
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician
[This message has been edited by stevebfl (edited 06-25-2000).]