MPC6500 random registration shift only on duplex side

[mrwho]

Good morning all!

I've been battling one of these machines (I really hate them!) with random variable registration shifts (image moves in the lead edge direction to the outside of the sheet) only on the second side (the side first printed is always ok, and also if I print simplex the problem doesn't ocur). Also the problem only happens with LEF - if I print in SEF it doesn't show the symptoms.

I've replaced several motors and sensors on the duplexing unit, and yesterday I replaced the entire duplex unit with another machine, but the problem is still there. This leads me to the conclusion that the problem must lie on the registration unit, although I fail to understand why it doesn't show the symptoms when printing simplex, as most of the parts that control the registration ar the same both for the trays and the duplex.

Any help from someone who already had to solve this?
Thanks!

[TonerMunkeh]

Could be a lazy registration sensor affecting the laser start timing. Have there been any laser codes?