neas- if the spur/pinion is the problem then switching to 64 oitch may make it worse, because to trash a 48 pitch gear that bad the mesh would have to be aweful, and 64 pitch gears are harder to set the mesh on. Have you check the tightness of your belts? what size pulley are you using? if the 17 tooth pulley is being used and the layshaft mounts are installed backwards then the rear belt will be too loose and cause this problem. Did it do this from the first time you used the car after building?