The rx8 pcb should be fine with any rpm motor. I don't know first hand which motors qfm sells but I'm pretty sure they're pushing bit style motors (with the small tab contact on the ehdbell of the motor). I had a problem with bit motors not making a good contact in the zips. Most times if you turn the motor in place so the tab is pointing towards the back of the car you'll see a boost. I'm pretty sure it's a contact problem but it could be a bum motor. Have you tried the motor in a bit or clone? What color is the motor?
QFM is here in the states so a refund or replacement should be easy if it's a bum motor.