MX...IF it is the motor and not a stripped spur gear, then upgrade to a brushless setup for sure. I went with a Novak Havoc Sport ESC/Motor ($140), and went to 5000 LiPo batteries as well. Kind of costly to do it all at once, but you will not be disappointed with the improved performance.
I just picked up the Novak 8.5 brushless setup and fan for $148. The Velineon VXL was $180 at my local hobby shop. I have had Novak brushless products in my other trucks and loved them.
The 8.5 setup is plenty fast for the Slash.
Ya i went to the hobby store today and it was the new spur gear. However i got home and installed the new gear and the motor locked up on me so idk. I am going back to the hobbie store tomarrow to find out what is going on.
The same thing happened to my stock motor, would just lock up for no apparent reason. went out and bought the Velineon VXL set up, problem solved and fast as hell. The only thing you might want to check is that the gear on the motor is not to hard up against the spur gear. Somtimes those spur gears are out of round, and if your spacing is not correct, it will cause problems.