I've got a '90 4WD 5-speed with 140,000 miles. It accelerates fine in 1st and 2nd. But I get a little slip in the clutch in 3rd when accelerating hard, and A LOT of slippage when accelerating hard in 4th and 5th. This happens when I've taken my foot completely off of the clutch pedal. The pedal appears to travel all the way back up off of the floor as well. I can accelerate gently without any slippage. Any suggestions on what the problem could be and the steps I should take to diagnose this?