I'm using the 5900 on a 60" GWIII that's been tweaked to the 9s.(lets talk color decoder, grayscale, enhancements, etc) Making adjustment to 3DNR and MQNR for all intents and purposes has corrected the dithering/noise that I was seeing. For now I'm distracted with thinking how good the picture looks and sounds to stay focused on the actual program.
What's interesting is back in February when I was auditioning a Denon 2900 on a GWIII, it had this same effect. Problem is the 2900 doesn't have the adjustments that the 5900 does. Anyway, the 2900 I bought didn't have this problem on my GWIII. Sounds like the firmware may have been different between the dealer's demo unit and the one that I actually bought or maybe it was broken.
Although I really liked the 2900, I'm glad that I upgraded to the 5900.