Bunged Up

Here's a video of my RepRap behaving as it should:



But for the last month or so I've been having problems. At random times the extruder started chewing filament instead of pushing into the heater. This of course resulted in failed prints. The first symptoms are particles of ABS accumulating on the idler bearing of the Wade extruder:

If allowed to continue printing, the extruder simply stops pushing ABS into the heater. On dismantling the extruder the hobbed bolt looks like this:

After cleaning with acetone it's pristine once again:

But after cleaning, sometimes I mange to get 2 or three large parts before another failure, othertimes it fails at the first print. I've tried reducing the flow rate of the extruder but this hasn't had any effect. The only consistent thing is that the print always fails on the second or third layer, never the first.

I got the nozzle from Wolfgang at RepRap Fab and I've bought a new one to see if this will improve things.