Alternative Way of Adjusting G2s (25EC58)

[old_tv_nut]

OK, you may have two problems combining to give this effect.
The change with fine tuning, channel change back and forth, could be a tuner issue. Then, as the change in tuning affects the color level, it could be some kind of overload in the color circuits related to the ACG.

First, get the fine tuning right. Turn off the AFT, and set the fine tuning manually. turning one way should make the picture blurrier and reduce the color signal. Turning the other way should make the color signal stronger. There may be little visible change in the color strength due to the internal automatic color control (ACC), but eventually on this end of the adjustment, the picture will show "wormy" interference that moves with the sound content. When you see the sound interference, turn back the fine tuning until the interference disappears. Once the fine tuning is correct, try adjusting the ACG.