I have what appears to be the original monitor in my Capcom Impress. I've fed it resolutions it couldn't sync to, and got the same as you did. Now, maybe the monitors before the mid 90s were touchy and would fry....
I don't find the capacity for higher resolutions via a jumper or switch on the chassis useful for a MAME machine, unless you're running everything at such a low resolution, in which case why use the monitor.
Anyways, now you know.
Consider it high comedy....sincere tragedy....whatever...don't take it personally.