Spend a weekend day and flush the coolant real good, make sure both the ac radiator and the cooling system radiator is free of debree. These cars like to pile it up and flush your coolant real good and change your tstat. Also check your fan clutch or look into getting the bomb taraus fan and be done with it.
Let us know what you do to fix it. Going to do one thing at a time or sort of attack it?
I do sometimes if all the parts are old and stuff why trust em? That's part of having a reliable car. While your stuck under hood might as well
You said your radiator, fan, clutch, and thermostat are fine. Did you actually test them? There isn't much left. The fact that your AC stops getting cold at the same time the cars engine is heating up points to an air flow problem. That is the only thing in common to both systems. The engine coolant temperature has very little effect on the A/C temperatures, the car could be overheating and blowing steam out of the radiator cap but as long as there is air flow through the condenser and the compressor is still spinning and compressing, you'll have cold air (assuming you're in the full cold position and the flap is not leaking too much hot air through the heater core box).
With low air flow, the condenser does not get cooled (device in front of the radiator) which means your AC won't work well and the radiator behind it does not cool down which makes the car heat up. Does this happen when you're are cruising at a decent speed down the highway or only when moving slow or stopped (under 15-25). If only when stopped, I'd guess the fan or clutch, if when moving at a decent speed and stopped, something is physically blocking your condenser and or radiator like bugs, rocks, dust, bent fins or your plastic air deflector under the core support is missing or bent out of shape or a combination of both some air blockage and some clutch.
The stock radiator, fan, and clutch is capable of cooling in 100+ with the AC going even with a decent amount of mods. An electric fan has advantages but if you have an actual cooling issue that is not the fault of your existing stock fan and clutch, a powerful electric fan will only temporarily hide the real problem.
I was going to add that if you have an electric fan and is was pushing air instead of pulling ,then it would be pushing the hot air from your radiator over your condenser coil which would also make your A/C not cool.