Narco of course are no longer supported. Normally when the displays go its game over as you cant get parts.
If it was me I would put the narco on the bench and let it run. Give it a few calibrated whacks and try some freeze spray. I am guessing it gets cold at altitude there. It may be that something is heating up and causing your U/S. Failing that you have a loom you can put the aircraft on ground power and let it warm up, make some calls to a relatively distance station.
I always do a VSWR check and if you can get 2 its great but 3 is acceptable.
Load test the Circuit Breaker to the radio a 22 ohm resistor in series with the multimeter and see if you get a voltage drop. If you do its the CB. These older radios dont seem so voltage sensitive as say the microair
Check corrosion on the pins and there tension. Also that its racking correctly. Also the screws in that back of the rack are present
Normally when flightcoms go its obvious from the start so I'm thinking its in the Narco.
To be honest if that's the problem junk it and get a SL40 or Icom. Fixing them is a false economy.
You mentioned you play with connections. If its the jacks they can lose tension on the prongs. Also if you have left the aircraft sitting corrision can build up on the jacks. A bit of scouring pad cleans it up but if so then replace jacks.
I guess you have a switch panel so check there for dry joints.
Ok thats a lot but hey good luck.