I have a 90 f150 4x4 5.8l. e4od. to get it to move in first gear it revs up then moves real slow then it will shift fine until I stop. then it does it all over again. if I manually shift in first no problems. fluid is full and clean new neutral switch and harness. what else could it be?

 

So. If I'm reading this correctly, it's starting off in some other gear then first? If so, check the connections all over the trans; the sensors may have a connection problem, however, that would usually trip a CEL.

 

Did you notice if the OD light on the shift lever was on or flashing?