[HI Cobra]

Hope someone can help me here.
Very recently my Tremec 3550 is having a problem staying in reverse.
It just eases out when I am backing up and the only way I can keep it
there is to hold it in tightly. Everything going forward is fine - no problems
popping out of any gear. I've never ground it, missed shifts or abused it,
and got it new with the Cobra kit. I have close to 18k miles - but that isn't
much for a transmission for the way I drive, which is mainly to the North
Shore and back, about 80 miles round trip and mostly on our freeway.
The clutch is fine = no problems with it either.

Anyone else encounter a similiar problem, and if so, what fixed it?
Thanks for your help.