My 42PD3000 has had exactly the same problem from the day I set it up. I contacted the supplier who suggested I use the screen wipe (which I'd already done) and to give it a week or so "to settle down". Some weeks later, still with a central cloud clearly visible on a supposed black screen, I contacted C&M again and was referred to Hitachi Customer Services - who in turn pointed me at the local service agents.
I arranged a service call under warranty and was visited by an "engineer" and his mate. The former then spent 2 hours trying to get the display into "service mode" (including phoning his office, and Hitachi), and finally left, having done nothing and muttering techno-babble to try and cover for his patent incompetence.
I then had to chase them, at which point they turned up and took the set away for "testing" at the workshop. A week later, I had to chase them again, and they returned the set - carefully bubble-wrapped and completely un-marked . . . oh, and completely unchanged.
I've subsequently run the screen-wipe for hours at a time, which does improve things - I'm pretty sure that the problem has diminished - or maybe I've just gotten used to it.
I weighed up my options and decided that yes, the problem is annoying when it becomes visible (like when watching the X-Files, which is mostly filmed in the dark), but 95% of the time you can't see it - and even when you can, it's livable with.
btw - it's really worthwhile turning off pretty much everything that Hitachi provide in terms of picture "enhancement" technology, and calibrate the brightness, contrast etc using a reference disc.
Bottom line? Hitachi have lost another 20 or so customers. I wonder if they care?
btw : does anyone know the service menu access procedure? It's something to do with setting a particular frequency on one of the channels . . . ??