Cougar sv diff and gearbox can anyone help

[maseace]

I've had the cougar sv for a few months now and race about everyother week, so far I have been through 4 diffs two gearboxes two spur gears, it is running a 78 26. Now first off I would think it is how they are built gearing etc but my mate is running the same and has had his the same amount of time and we build them together and he has had no problem can anyone think of any other reason the could be happening? I have also ask other people about the slipper and have had it checked and set up all ok on that front. I'm lost and thinking of just selling it now