Mystery problem e4od 94 5.0 4x4

I can drive this for several miles and then tranny slips and won't move. I shut off for couple of minutes...start it up and everything shifts fine for 5 10 miles and then slips again. Reverse shudders since it was rebuilt about 25000 miles ago. I had it rebuilt at a Tennessee shop, emergency situation so reverse gear wasn't corrected right away as anybody should have but had to get back Iowa. Any way i can drive it for about 50 miles and then it slips...shut off and it works again. ??? Is it junk???? electrical issue??? thank :runaround: