5si registration problems.

I purchased an HP 5si (used) and it seems to print acceptably some of the time but the rest of the time there is a diaganol line of the textstarting about 3/4s of the way across the top of the page running to the right edge about half way down the page. After searching around I discovered that this seems to be a registration problem cuased by the registration roller.
My question is will simply replacing the registration assembly fix this problem or is it something that needs settings tweaked? How can I get this to go away so I can enjoy this printer?

Thanks in advance

Neil