Not really sure, but, maybe electrical interference?
Maybe the position of everthing in that particular car is causing some receiver intereference? 27mhz is more susceptable to probs with that I think...
Try moving things around, separating the recevier from other things better?
Also, you say the spur is not concentric...how well meshed is the pinion?
You'll need to adjust the motor/pinion position so that there is a tiny amount of play between the teeth at the closest/highest point on the spur...not sure if the amount of meshing would cause the stuttering you've described, but adjusting that will help the general running of it. It's a prob with some spur gears, best fix for that though is to buy a new one and hope it's got better concentricity. Be careful though...give it too much play and it'll start shreading teeth at the low point on the spur!
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