I was once told to ease the front end of my car up to a large pole or post and stop with the bumper against it. Put it in first gear and let the clutch out slowly. If it engages and dies, it's good, if the clutch pedal is let all the way out but the engine keeps running, it's shot.
Don't know if that applies anymore...
Do the RPM's increase during the loss of power? If so, I think the clutch is slipping.
2006 4.0 5 spd auto 4x4 Super Cab
BF Goodrich Commercial Traction T/A's
Royal Purple Throughout
Alpine Head Unit with some Infinity Noisy Stuff