Oki C831 white streaks / bands through colours

[sunspitter]

I purchased an OKI C831 earlier this year, second hand. All of the consumables are completely topped up, the life span of all of it at near 100 percent. It's given me some really great prints in the short time I had all four colours topped up. I gave everything a clean and swapped out a magenta toner after about 2 months of it being empty and not printing any full colour pages (only printing B&W), and this problem developed out of nowhere. This issue wasn't present when I bought it, nor is it present on some colours when printing, but when I print something in full colour bands run through the prints and full colour prints often come out incomplete (e.g. first quarter of a page is printed then the rest is blank).

I was told by OKI support to print the individual CMYK colours on different pages, the results of which I'll show here:

https://imgur.com/a/cZZOgNs

I was then told to clean the LED heads and their contacts (which I've done) with no success. The problem area is the same on all 4 colours.

Is this a case of replacing the LED heads? Or the Fuser? Seems such a shame to do so when the diagnostic says they're okay.