I had the same problem with a 904 I built .they put it in ,took it out and drove it it was locking up in 3rd .I went to the shop and put a pressure guage on the front servowhen it shifted into 3rd it never released the band I checked the linkage adjustment it was off. I put the unit in 3rd and adjusted the cable so it would go right in the problem was fixed.but you know how it is the trans builder is always at fault lol.hope this helps.