4l60e no 3rd

I recently picked up a 2000 Escalade with a spun main bearing. Tranny was supposed to have been rebuild. Tranny was clean and painted freshly so I believe it was rebuilt. Put in fresh rebuilt engine and take it out for a drive, all is fine, shifts right, everything works as it should. Has only been used for short trips, 5 miles at a time. Got about 50 miles on it, 3rd has a slight slip to it, 1, 2 and 4th are fine. Let it sit overnight, its fine again, when warmed up, a slight slip in 3rd. Then 3rd is gone one day, just like neutral. I drained fluid, it was burned, tossed in a new valve body with all new solenoids and everything, no difference. Goes into first, then second, acts like neutral in third. Place shift selector into 2, it will go through first into second, get to 45 or so, place back into D and it will run in fourth fine with no slippage. Any advise is greatly appreciated.