I've had this on my drivers side wiper and it was caused by the cable breaking....well not completely breaking, I'll try and explain.
The cable that runs from the wiper motor to the 2 wheel boxes is made of 2 bits, there is a inner cable (multiple core cable) which has a solid wire wrapped around it very tightly but spaced out to make what is effectively a very long flexible screw. (Think of it like a flexible rubber brake pipe with the black plastic spring like thing wrapped around it).
Anyway, what happened to mine was the outer cable snapped and would move back and forth on the inner cable, so you could push it back and it would work fine for a while but it would slowly creep further and further over until it got stuck over the side of the windscreen.
I had to replace the whole lot...I did have major issues because britpart sell a replacement cable (fairly cheap) but the pitch didn't match the wheel boxes so it was a waste of money, I had to replace the wheel boxes also. Did have another issue because I bought a secondhand TD5 assembly off ebay but didn't realise that they are shorter on the later models as the motor is mounted in a higher position!!