VT-71 'rolling band'

[miniman82]

So I decided to take a break from the CTC-4 to give the VT-71 more love. It's always had sync issues, and there could be a little more width. At first I thought there was something wrong with AGC, but it turns out to be doing what it's supposed to. Then as I was scoping waveforms, I noticed that when it would lose sync the waveform would grow slightly. Looking a little closer, there's a damn band rolling through the sync waveform and when it reaches the sync tops that's when the picture tumbles. It looks like a positive half sine wave to me, but a small one.

Power supply is the most likely culprit for a rolling band, but the power supply looks clean as a whistle to me. So is there anything else that can cause this issue in these chassis, like a signal beating or something? It only seems to be affecting the horizontal section, vert is solid.