First thing you may want to do is to put all the stock stuff that came with the car - motor, drives haft and take the heatsink off.
Then with it set back up to how it came from the factory give it a run and see how it goes, if it goes all well then install one thing at a time, eg- heat sink, drive shaft, 15t motor.
Without seeing the car my guess would also be that the 15t motor is drawing too much current for the speed control to handle. If you want a better motor for drifting then I'd reccomend starting off with a Tamiya "Sports tuned 540", it's basically the same as the standard motor but with a little bit of extra power.
The next upgrade you should look at after you get your problem sorted is a set of ball bearings for the car, these can be bought at various hobby shops and online stores. The ball bearings will make your car run smoother, faster and will have less drag on the rotating parts etc.
Hope you get the probs all sorted out
Cheers
Rob.