That issue is caused by a weak sync mostly. The TV will take any dark image as its sync, but if the picture goes too white, the TV will lose the sync, thinking the voltage is waay too high(but it's in fact lacking sync).
A 100% white picture, according to NTSC standards should be 1.00 V.
I once read that the absolute admitted value is 1.073V, and anything above that will cause the TV to ignore the input signal.
BTW, I told Guntz to use white screens as reference screens for voltage measuring, because a pure white image is made up of 33.33% of Red, Green and Blue. This applies to Luminance too