try taking the pinion off the motor,keep the spur in so all the transmition as working,push the car along and it should keep going,if it slows really quickly then something is binding,this will melt the slipper,take out the spur and roll the front wheels then the rear to see if its the front or back,then work your way down till you find it,it doesnt need to be really tight to melt it,i had a sweet rapper stuck just under the front drivshaft just before the gear box,it was enought to bind and melt mine,its never don it before and its never done it again,sorry if you have done this,i hope you get it sorted as the car is ace
ps just check all the bearings because they are(sorry)really crap,