One further clue - it seems to work OK if I only print a few pages at a time. If I print something that's say 50 sheets of duplex printing (100 pages), it starts to go wrong somewhere between 10 and 20 pages in. It's almost like the fuser runs out of steam after a few pages.
Yes, the print rubs off - which is why I tried replacing the fuser.
I tried your suggestion and the tray is already set to plain paper (which is what I'm using). I tried setting it to light cardstock but the printer properties on the PC appears to be over-riding that. If I set the printer properties to light cardstock, it then refuses to do duplex printing which isn't much help to me as I need a duplex printer (there's no way I'm printing 100s of pages with manual duplex). Is there any other way I can persuade the fuser to do a better job?
That would, if it smears when you touch it (rubs off) mean it isn't fusing.
That would imply the fuser has an issue, or the toner is not right (has a different melting point) then the fuser requires. But...
The fuser adjusts its temperature according to stocks loaded, heavier stock means a hotter fuser to bond the toner to it.
So if you are running plain paper and the tray was told it is lightweight, cardstock etc ect, the fuser will be at the wrong temperature for the toner and stock. This can work for, or against you.
If it is loaded correctly and does it, try incorrect settings to compensate for the issue at hand and see if it comes in line.
If it isn't properly, set, fix it.
To set the paper type for paper in the tray:
I have a Workcentre 6505DN that I purchased in October 2015. It only gets light use. It started smearing just over a year later (example below). It appears to be dark areas of photographs that are failing to adhere to the paper. I've tired various different sources of toner and I've tried a variety of good quality copier papers. I've also tried replacing the fuser unit. Nothing has made the slightest bit of difference. Any suggstions would be welcome. This printer is pretty much unusable at the moment, so I either need a solution or I'll have to bin it.