Fowkc, I was getting similar problems with my old laser too.
So, at great expense, I changed printer and the media too and that sorted most of the probs. Expensive, yes, but then so are a lot of mucked-up photoresist boards. The only problem that remains are large areas of copper where it gets patchy in the middle.
(I know why, so I don't want a lecture, but I can't afford the type of laser that will give perfect results).
A black felt-tip gets around that.
I see Rapid Electronics, amongst others, does a "specially formulated" coated film for lasers at 50p per sheet - never tried it, but if its any good and it saves on FR4 then I'll get some. Anyone tried it (or similar?) Its (apparently) not the same as laser OHP film.
Incidentally, has any tried those cheap CNC-type drilling machines such as sold by Milford Instruments? I'm tired of drilling 400 holes in a prototype board.