Recently found the same with my C3000, after one year of flawless fun.
At first appeared like an intermittent contact with the display.
Also, when I run ZColorAdjust, the reds are totally wrong. Green and blue are correct.
Tried about the same steps as you did, to no avail.
Gonna dig up intermediate backup images to see if it is not resulting from some install.
Ill let you know if something turns up.
Cheers.