Being a 91, your A/C came with R-12 Freon. Has it ever been retrofitted for R-134A?
My first thoughts would be that the clutch is kicking out on you OR a blend door is losing the required vaccum in order to keep the hot air from the heater core out of the flow.
When the air warms up, does it ever move the vents it comes from, like from the dash vents to the defrost. If not, I would lean more to the clutch kicking out. There is a switch that turns the voltage on and off to the field coil magnet on the clutch. And I think that your system uses a expansion block metering device not an orfice tube.
When it gets warm all of the sudden again and while it is still warm, if you can catch it doing it while in park, check to see if the clutch is engaged when it's acting up, it it is still turning, then I'd look at the blend door/vaccum lines.