I've barely worked on scopes so I'm just guessing.
Would it be some HV problem, where it takes a while to build up enough voltage, maybe on some cap, or to tun on some BJT, and then gets drained away and repeats ? I'd check all the voltage rails too, maybe the supply can't keep up with the HV for some reason.
Or maybe it's a sweep problem ? I can't remember the sweep's on very long time bases on my Tektronix CRT scope, but it looks more like your 20ms trace, which should be sweeping way more often.
Here's some schematic's, for fun I'm looking at some of it, but yeah there's plenty of cool circuits that I'm not that familiar with, when you tie them all together.