The final stage resistor should not affect temperature from the vents.
It sounds like you have a low charge. To diagnose, need a tool that can do live scanning of the AC system. Or start with a system low side pressure check. It could also be a compressor clutch getting worn. When they get hot they stop engaging. If possible, when the air goes warm, pull over and see if the compressor is spinning or if the pulley is just spinning without turning the compressor.
At the mileage your car has I bet it's the compressor clutch.
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