Tek 475 HV issue

[miniman82]

I just picked up a Tek 475, and it's got an interesting issue. After being on for about a minute, HV kicks off then cycles back on all by itself repeatedly. I've been poking around in the circuit, and I can't see anything that would cause it to cycle like this. The regulation circuit is working correctly and the HV is fine when it's on, but for some reason it kicks off. Anyone have any idea what may be causing it? All the waveforms and voltages in the manual check out, it's like it's switching itself off and then back on for no reason.