Good idea looking at the bushings and replacing them with bearings. If you remove the pinion gear and spin the rear wheels, does everything spin freely?
Also, when you check how tight the gear mesh between the pinion and the spur gears is. make sure you check all the way around the spur especially if things have heated up as it could have warped.
It could also just that your battery has a bad cell- in that case it would only be a new battery that would fix this problem.
Also, if you've got the optional pinion gear, make sure you are running the smaller gear!