Problem p5000etch SNF 2008-01-28 18:42:35: Trouble with robot arm

I ran 12 dummy wafers in 3 lots (with 4 wafers per lot) in Chamber C. The outcome was:
1) First lot etched fine, but gave an error "Cannot complete: Robot extension position is not known".
2) Second lot didn't enter the etch chamber C at all. The same error as above showed up before the wafers could be loaded into the chamber.
3) Third lot proceeded fine upto 2 wafers and then the third wafer was not loaded into the chamber C because of the same error as above.
Upon checking the error log, it appears as though the encoder on the robot arm motor has problems.