I believe the AMS also has a part in feeding during printing and not just getting it to the motor in the printhead. The printer has a small filament buffer that the AMS tubing is connected to. As the printhead takes the buffered filament it is the job of the AMS to push a small amount of filament to replenish the buffer.
A stripped tooth on a drive gear could affect at low speed but when running at a slightly higher speed the extra inertia could overcome the deficiency. Same with a bad tube, the drag would be harder to overcome when only a small movement is required but not so much with a longer feed.
Whatever the cause, once I have finished the parts for the current ptoject I’ll strip the AMS and take a look.I’ll probably purchase a new first feed assembly as well as PTFE tubing regardles just to have them available when they will inevetably needed.