I am racing TT Track with my DS650 2002 Bombardier. When I am under full power it jumps between 3rd and 4th gear. I had the whole transmission rebuilt last summer for the same problem and it still does it. Has anyone had this problem and do you have any ideas to help????

 

First of all welcome to the forum first timer.
It sounds like you have a bent shift fork and/or a worn dog (no, not ol Red...he's ok). The dogs (enguagement dogs) are the little ***** or **** that stick out of the side of the gear that enguage the gear next to it when you shift.
When your trans was rebuilt, they must have missed a slightly bent shift fork and now the edge is worn off the dogs because can't enguage fully with the mating gear. Every time it pops out of gear the dog becomes slightly more rounded until it won't hold the gear at all. Whenever a gear is replaced due to a worn dog, the mating gear must also be replaced or you're just throwing money away.
This time replace all the shift forks and both gears. It will be money well spent.

 

Get new dogs get your gear under cut then send out all your tranny parts for Cryogenic Tempering