Hi Martin,
Have not encountered this as I run both motors from a single ESC but if you swap the esc's to the opposite motor, does the problem stay with the same motor, or does it switch to the other motor. If it is still the same motor, the problem could be that motor or prop linkage. If it switches to the opposite motor, then the problem is in your esc. or possibly the wiring to that ESC. Just food for thought.
Norm