Thanks very much for the replies guys - definitely some food for thought here... I will try swapping the motors and gearboxes over and see what happens.
Each gearmotor is connect directly to a wheel and are mounted on opposite sides of a small robot. I was initially testing the stall current by applying a breaking force to the wheels - i.e holding onto it
- I noticed the problem because when moving across the ground one motor was stalling for no obvious reason and the current consumption was skyrocketing.
Anyway, now I have the motors completely dismantled from the robot chassis and the wheels removed but still see the same symptoms.
There doesn't seem to be any springs or clutches inside the gearbox - it's not fancy... just gears, spindles and bushings and a lot of grease. I will see if I can grab a photo later.
I was wondering if some manufacturing issue with the motor windings or geometry could explain this. I have some more motors on the way... I'm hoping I can find a pair with roughly equivalent performance!
Phil