my model is used, i bought it off ebay and has never been working fine.
I used to own one back when it was released in 2012 and that one was an absolute beast even without mod's.
Back then i thought the problem was that the motors where bad so i bought new ones, that didn't help so i replaced the IR reciever and the battery box.
That did not fix the problem.
Changing the tires gave it back a lite Torque but not as much as it used to have.