AC Troubles… it’s blowing warm-hot air

I picked up a ’01 Tacoma with a few miles on it (262k) and one of the issues with the truck is that the AC does not work. Here's what I do know:
- When on defrost, it’ll blow cool air out of the side and windshield vents.
- When using ‘vent’ with the temp knob on the ‘max’ cold side and other knob on Head, Head/Feet or Feet, it blows out warm-hot air through the vents.
- I can see that the Fresh Air/Recirculate trap door works, if I tip the glove box down.
- When turning on the AC, I can see the clutch engage on the compressor.
- The fan does blow in all 4 settings.
- The Freon levels are full, according the 'top off' AC recharge bottle.
- The Heater Valve opens and Closes when adjusting the Hot/Cold Temp control knob.

If I play with the knobs and buttons (no specific way), I get a blast of chilly air for a couple minutes and then its hot again.

Not sure where to begin to troubleshoot.
What things can I look at before getting a mechanic involved?