ok, i see.... it seems there is a tiny bit of synchronization jittering going on, but I'm not really sure where you might start looking. not sure if this is an original pacman game board or if it's an xxx-in-1 type deal, but it could be your game board having a variance in timing or something as it seems to be doing it in a vertical direction to the monitors scan direction. maybe a power supply introducing noise? could just be a bad ground somewhere give the video cable a wiggle and clean up the board contacts and make sure everything has a good connection.
it could be something in the monitor. there is a timing capacitor for the vertical deflection circuit. it's gonna be a small (like 100uF 35v (C9?) near the vertical deflection IC) cap that's responsible for the main timing ... it's constant charge/discharge being the "heartbeat" of the timing for the vertical circuit to go to the next line. if it's starts getting old and flakey, it can cause jitteriness... it's usually much more noticeable than this and you end up with weird overlapping and foldunder and stuff so it could just be marginal.
it's not something to really worry about. i mean it's not doing too much... just a little annoying if you stare at it looking for it. if you have a screenful of stuff moving around and you are playing it, you aren't going to notice it.
if it's really getting to you and you really need to do something about it... if you have another monitor or another gameboard/power supply you can try... see if it's coming out of the board like that or it's the actual monitor doing it, becasue honestly, it could be either one doing it from what i can see.
if not, and your monitor has never been serviced, maybe start with a cap kit for your monitor and see if it still does it. they are pretty cheap insurance for the future and will make that monitor outlast you... but will also MOSTLY eliminate the monitor as a source of the jitter.