I have a II+ (rev 07) and I've found that when I adjust the pot to a more intense signal, the color killer in text mode abruptly fails and I get green/purple bleeding. If I adjust it even higher, the green/purple switches to red/blue.
The color killer only works for very low intensities - hardly bright enough to be usable.
I've tried replacing 74LS259, with no effect. Any ideas on what else I should check?