Is the light path in your enlarger clean? Lamps, condensers, filters and diffusers can accumulate dust is storage, and stopping down increses the depth of field so that what you don't notice on focussing at f2.8 could easily affect the print at f8. So you could easily have a film of muck on surfaces you wouldn't even notice. I'm not saying that's a definitve cause, but it won't hurt to check it out.

Good luck and cheers,,