Brother Laser output getting feinter by the page

Hi all,

I have a Brother HL760 that has recently started giving an output
quality problem. It has a new toner cartridge and has printed maybe
5000 pages, mainly text, in its life. I have tried turning up the
contrast potentiometer inside.

I am trying to print graphics, for PCB making with UV light, and need
high density black. It's always performed well before.

Now, the page starts off great, but a quarter of the way down starts
to get fine bands of feintness across the image. If I print multiple
copies, they continue getting progressively feinter until the 3rd copy
looks decidedly shoddy.

If I then print again, the first page starts OK but repeats the above.

If this is just a drum problem, I will replace it. But the drum is
£100 and I can't afford to waste the money if it might not work. I
don't think it is the drum myself, but what do I know?

Any ideas, kind people?

Rick.