Canon IRC3220 Scanner Reg Off

OK, this is weird. If you put 3 or 4 sheets in the feeder everything is fine. If you put in 10 or so, about half way through the stack the scanner starts to scan the right half of the glass instead of the left.It finishes the stack this way.
The feeder is putting the originals on the glass correctly, it's like the scanner looses home position or something. No codes, no bad noises.I cleaned all the sensors I could find and checked belt tension on the drive motor.
This is a new customer that bought this refurbed from somebody else. Any help would be much appreciated.Thanx one and all!!