Wobbly like the picture has a wave or ripple going through or "under" it? I've had this happen on two monitors, once it was because the B+ pot was faulty, the other time was because one of the larger caps near the HOT was faulty. I'm not an expert though, so I would wait till one of the more experienced people here chimes in.
If it's only in certain games, especially with GroovyMame and if you're using CRT_emudriver then it could be some manner of resolutions/timings that your monitor isn't doing well with. I had this happen a lot with GM and CRT_emudriver. Mortal Kombat in particular runs at a lower refresh rate and some monitors have trouble syncing correctly.