Ok so this is an old thread but I thought I'd do an update.
I have been putting up with the belt clicking at the end of the purge stroke all this time , and of course worried that one day teeth would go missing from the belt.
I wanted to do a print yesterday that entailed using "pause" so I could drop in an insert, but the belt click meant that the printer would resume in a different place.
After hours of trying to figure out what was wrong in the software/firmware I suddenly noticed something.
It's like this : The original filament feeders are poor on these machines so I printed a couple of new ones off thingiverse that use a spring for tension. They work really well except for one thing. The levers stick out to the side of the feeders so you can push them down to open the gate. The lever on the left feeder hits the end of travel before the X position has finished counting out, and so it stops the X travel while the stepper is still running and hence the slippage.
Now I have to make repairs to the left feeder lever and problem finally solved.