I ran across several trucks that starts, runs fine but won't do anything other than idle or runs fine wide open then 2 minutes later, it will only idle no matter what.

Check the battery and alternator output voltage. Most likely the belt is slipping on the alternator pulley. Probably only has 11.5-12 volts output.

Tighten the Alternator Belt and recheck. You should have now a constant 14.3-14.4 volts. Problem solved!