1995 Sierra stuck in limp mode
 

I have a 1995 GMC Sierra 1500 that seems to be stuck in limp home mode. Occasionally it will shift great through all gears smoothly and has for up to 60 miles at a time. Mostly we have 2nd, 3rd and reverse. The fluid is not low or burnt/dirty and we have replaced the TPS and temp sensor. The PCM is new and has not thrown any codes so I'm at a loss. Anyone have a clue?