Based on the fact that your RPM's drop like the truck is going to stall, I think there is a fair amount more at play here.
It's possible that the AC compressor is starting to go bad and it's causing a massive load on the engine.
I would suggest two things:
It's possible that the AC compressor is starting to go bad and it's causing a massive load on the engine.
I would suggest two things:
- Check the amperage being pulled when the AC is turned on - High amps = high load - Could mean compressor is going bad and it's physically slowing the engine down when it engages.
- Check for air leaks with smoke - Air leaks will cause a serious amount of weird issues, but I would think you might notice some other drive-ability issues is it were that serious.