I've heard that MRRC cars can be pretty torqy, which usually means more amp draw than usual. They make rally cars and parts, after all, where torque over RPM is better. If you chip it with any Scalextric brand chip, test it first with the wheels up by going at different speeds and different amounts of acceleration, up to the point where you're pinning it for a second and fully releasing it over and over. If all is well, try the car out without any magnetic traction. Run a good 50 laps if you can. If that's still good, then you can add magnets if you want, but start out with just a little at first. The more drag you create, the more amps the motor will need, and the more likely the chip will blow.
If at ANY point the motor stops or slows unexpectedly, even if during the "wheels up" test, then the chip is being overloaded. If that's the case, you have three options:
1) replace it with a slot.it brand SSD chip. They can handle more motor
2) add a second chip in parallel to the first (only one LED is needed for lane changing, though). This will split the load between the two chips, and it will work better
3) upgrade the chip's mosfets to ones with a higher power limit. RichG has a thread on this if you're good at soldering tiny things, or you can have someone else do a chip for you. I know that gmyers does them at a reasonable price, and I know others have done them both for themselves and other people, but I can't volunteer on their behalf. ;-)