Try this: best thing that I've found to reduce the number of times you have to rebuild your diff is to keep a small compressor with you or a can of compressed air at the track. Just blow off the fuzz after each run. If you allow the fuzz to accumlate it gets worse.
I was never convinced that dust covers really work. Kind of like dust sheild on ball bearings. For some reason dirt always seems to find a way into the bearings and gets trapped. I would think te smae would happen on diff covers?
If you diff feels gritty, try relubing the small thrust balls first.
Ran, my 415 last night, switching to Parma Cyan/Maganta's vs. Plaid/Purple. The only change I made. Both set-ups yield almost the same fastest lap, but the Cyan/Magenta set-up was a lot more consistant.
Then switch springs from Tamiya Purple/Gray to ASC Purple/Copper with the Cyan/Magenta. Still had plenty of steering, but the car was not as twitchy. Makes sense as the stiff springs helped smooth out the car. Everythig else was basically stock set-up from the instruction books.
Except for:
front A block
Low friction belts