Monitor focus: Sharp in the middle, but a bit out of focus on top and bottom

One of my medium res arcade monitors, I notice, isn't sharp on all points of the game. I see that it's very sharp in the middle, but the top and bottom isn't quite so. Think of it like convergence on a rear projection. You can see the letters not lining up quite so well. What gives? Besides the focus pot, is there something else that can focus this better? More uniform?

Sounds like it is the convergence.

Picture please

I'll get some pics up later on when I fire up the machine. Think of two letters:

WORD
WORD

Only the don't match up perfectly. Like one of them throws up a purple shadow. This happens on the tops and bottoms... mildly, but the middle is nice and sharp. No idea how to fix convergence. The over all sharpness knob doesn't fix it.

It has nothing to do with focus.

Does it look as if the yoke has moved, or the magnetic rings on the crt neck ?