I hate Tamiya three speeds

[Lil Giants]

I have trucks with literally hundreds of miles on them in gruelling conditions and they shift perfectly till this day. I use the stock servos that come with the radio and have never changed a servo either.

I chuck that "servo saver" that comes with the trk kit and use the servo horn that comes with the servo, shortening it to what the servo saver device would be. I use the ball and plastic socket head on the servo and drill thru the plastic on the other end to screw the bolt into the locking collar on trans. That way the shift rod can't twist in opposite directions like with ball/sockets on both ends- messing with the necessary distance between shifts. That could be some of your trouble. Proper radio settings would be the other side of it.