Going thru gear sets is a weird situation. I went thru a streak where gear set changes were a frequent occurrence. Never was able to pin point the reason and presently am having no problems at all.
The main causes seem to be hitting stuff and extremely tight diffs. There are other causes, but those two seem to be the main culprits. Worn motor pinions is another frequent cause, but that is mainly a "newbie" mistake. Another one was a car where the gear shaft holes in the chassis had become loose.
The obsession with running a "tight" diff is unwarranted in most cases. The diff only needs to be tight enough to stop the "diffing out'' action under acceleration. When you consider how much torque a Silvercan has, the diff does not need to be nearly as tight as many suppose. Diffs that are too tight have many undesired characteristics that should preclude their use.
Super tight diffs were much in vogue when Minis were all the SWB version. The inherent push, there's that word again, of a tight diff was thought to be helpful. Like all things, time marches on and tight diffs no longer are what seems to be favored by the quicker Mini racers.
Again, the above is only offered for your consideration and comment and is in no way intended to demean or insult anyone.