Sticking Wipers?

I've noticed lately that the wipers on my 2003 Focus ZX5 (North
American) are behaving strange. If I turn them off in mid-wipe, most
times they simply just stop whereever they are on the window. They
don't finish their current wipe and go back into the hood.

Same thing with the "one touch" wipe. If I flick the switch, the wipers
usually stop where I let go of the wand, without finishing the wipe.

If I bump the car however, they then finish and go back into the hood.

Next time I'm in the dealers I'm going to get them to check this (and
hope I can duplicate the problem) but I was just curious if anyone else
had the same issues.