Volvo Repair: 1990 Volvo 740 AC problem, orifice tube, uv flashlight

Question
I've begun to have problems with the AC-First  incident was in May- had driven a hundred miles with no problem, but when  I started back, there was no AC, no power windows, no turn signals.  Figured it was blown fuses, so replaced them- still no AC but at least could roll the windows down. and use the turn signal.  Put car in shop- wires had melted, they  fixed the wiring, said the AC compressor was bad, replaced it, evacuated and recharged the system- everything seemed fine for a month or so. Last month made another  150 mile trip, lost the AC on the way home. After that it seemed to work intermittently. Put it back in the shop, and it worked fine for a day. Then I  took another trip- AC worked fine until I got on the interstate.  It blew out  hot air until I got off the interstate, then blew cold again.  Same thing happened on the way home- AC  until I hit the interstate, hot air on the interstate, cold air on the back roads.  It seems the AC  only works properly when I'm driving below 60mph.  What is happening and how can it be fixed? Driving below 60 on the interstate is not an option- I'd get run over.
Thanks for your help.

Answer
A couple things could be happening...The metered orifice tube, or expansion valve, may be allowing the system to get too cold and freeze up.The condenser and radiator faces should be checked for debris clogging the cooling fins.Compressed air is necessary to blow it out. Some models of that era had a circuit board in the AC controller that would develop a poor connection and wouldn't allow the compressor to turn on.Sometimes intermittently.Or the system may have sprouted a new leak somewhere.When the system was last repaired a leak detecting dye should have been added to determine the location of any new leaks that may develop.You can't see the dye without a UV flashlight, you should return and have them recheck the system for leakage and check the function of the controller and metered orifice tube (expansion valve)The controller shouldn't need to be replaced just re-solder the offending connection.