I tend to believe that service menu access has something to do with this, I never accessed the service menu, I let the tech do it, and on both of the G10s that I had, there was a black level jump after the tech was there. On his second visit (and my second G10) I mentioned the black level jump and how I believed that it was a combo of service menu access and hour count, he didn't have a clue what would cause that and proceeded to check for the DSE like streaks in the AR filter of my set. The next day I called him back because that night that he was there, I noticed the massive jump in black levels. He came back out and decided to replace the A board just to see what it would do (he was hoping to fix the streaks with it, which he admitted was a very slim hope, my loose s-video slot was just the excuse) The full factory reset is what fixed it, and I should be back into the 300 hour range in a few weeks to see if it goes back up again.
This is a bug in the firmware, and Panasonic needs to fix it. I would get another G10, but I am 100% sure that I will have this problem a third time.
Everyone having this issue needs to bug the heck out of Panasonic until they fix it.