Streaky Laser Printer Toner Cartridges

I've posted this question here because I think there might be electonics
element or some people here might have repaired a laser printer with
this problem.

Lately I seem to be pestered with streaky printing from laser printers.
A mate brought in a HP4M that was streaky on any toner cartridge used in
it, but the cartridges performed well in another HP4M.

So, I took the machine apart and gave it a good vacuum out (clean up)
and wiggle of cables and that improved things because the streaks now
took 20 pages to appear, instead of 4 pages.

I was also investigating whether there might be a few capacitors that
could be replaced to see if they were playing a part. There were three
electro caps on three different boards and that was it.

Now the disease has spread to my HP5si (don't put sick printers near
good printers).

Does anyone have any ideas based on experience?
I'm getting an almost solid black one inch wide streak down the middle of my printout and a similar but less dark streak down the rh side too. I don't have any operating instructions so I'm at a loss to know what to do about it. Any advice welcomed. (The cartridge is almost new).