if you have changed the controller did you reset the ESC too the controllers end points and centre?
does sound like gearing if it takes off fast then runs out at top end, what pinion spur are you running? (assuming its either 32dp or mod1)
what sort of temperatures are you getting out of the motor, if its running cool try getting a bigger pinion too raise the top speed a bit, just make sure the motor does not get too hot (if you cant keep your finger on it then its too hot
)
also what battery spec are you using......1/8th electric has alot of variables unlike the 10th side, try using a more powerful battery 4S?