uli Well, I spent more than an hour trying to replicate this, and after lots of tests, I think this is probably a problem of the H3 with certain LCD panels. I'll explain:
I have two identical monitors. Same brand (HP), same model (1740), but different panels.
I've used both with RPis 3 in the past, and one used to give me problems when switching resolutions, and the other didn't.
So, after trying
SCREEN 19, setting
CONFIG 10,1 and
SAVE CONFIG in both (then disconnecting the OPi and reconnecting), I could only consistently reproduce the problem in one of the monitors, and the other gave me no problems, it only glitched the first time, when I switched from one monitor to the other (probably some HDMI configuration stayed in the OPi?), but behaved correctly from that point onwards. No matter what I tried, I could not make that monitor to glitch the way I could the other.
I can confirm that, when the the first monitor glitched, I disconnected the Opi from it and connected it to my actual working monitor, a DELL U2412M (which, by the way, also has a DVI connector), and could see the video signal without a problem. So, it's something specific to that stupid panel and its weird DVI implementation. Least to say, I'm not using that monitor anymore.
So, I think that closes this issue, at least right now. If I find anything else, I'll let you know.
Edit: Having to disconnect and re-connect the boards turned out to be very tedious. Is there some command similar to Commodore 64's
SYS 64738 that resets the system that I'm not aware of? It could be very handy if you want to confirm that the configuration you just saved is being applied to the system.