SP5100DN adding blank pages

[Oldslap]

I have a 5100DN that is adding blank pages to some double sided print jobs. Also when you print the usage page instead of 2 pages you get 4 and some of page 1 is printed on the end of the 2nd page out of alignment and also not readable (just random characters).
I have upgraded the firmware, swopped the main PCB, HD, nic PCB, fuser & duplex fromanother machine. The reg sensor flag was worn so i thought it may be a timing issue, I fitted a new sensor assembly but the fault was still there.
Any suggestions?

[Eric1968]

Isn't this a case of multiple feedings? Maybe an idea to check friction pad and/or rollers?

[Oldslap]

Thanks for the reply,but no isn't multifeeding. It seems as though the printer doesn't know the correct page length. Items that should appear on the first page of the usage page are printed at the bottom of the second page, they are also out of alignment and not the correct characters.
two further blank pages follow. If you print a double sided job it occasionally does similar.

[jonhiker]

what about a problem with the memory?
also. have you hooked up a laptop and performed some of the print jobs?

[Oldslap]

Yeah, i've printed a double sided print job from my laptop and some pages were added with some print at the bottom of them.