Black streaks on copies from bypass tray and tray...

Question

I've had tons of people trying to figure this problem out on my Gestetner DSc435.
We are getting streaks of black on copies that are printed on heavier stock, i.e. 65lb. and even on lighter weight paper. We've changed the fuser and the black drum on many occasions and we still have the problem. It's strange; however, that it doesn't always do this...especially on paper smaller than 8 1/2 x 11.
Can you shed some light on this for us?

Thanks, in advance for your help!