Re :allison 1000 series

I have a chevy 2500 hd with a allison 1000 series in it .I bought it used with 70k on it .It looks like it was taken care of but i seem to be getting a trans problem that i want to get fixed before i pull anything .At 60-65 mph cruising along the highway ,if i decide to overtake someone and downshift it shifts for a second then kicks out and goes into a lower gear and the service engine light comes on .I read the codes and i get a 0735 which is a 5th gear ratio code .It seems like it see's a slip for a second and disengages .When i bought the truck it had a banks exhaust and filter kit and i phoned the origanal owner who had put a diablo program in it .The dealer i work at does have gmc ,but they do little repair to allisons and that is the reason i bought one .I cannot say for sure but i don't think that the upgrade is causing this as it happens without gunning the truck hard .If you think it is a good idea i can have the truck returned to stock at work .Well i hope you can help later brit .
Re :allison 1000 series

Does your dealer have the diagnostic equipment needed? If so I would perform a clutch test on it to see if it is slipping. At the very least I would do a transmission service. Remove the pan to see if there is any clutch material in the pan. Also, remove the spin-on filter and check the magnet for clutch material. If ther isn't much clutch material it could be something as easy as a speed sensor. Or a soilenoid.