The problem appears to be this line in mm.js (I don't know, there may be a similar problem in nn4.js and ie4.js)
pixMenuLeft = (screen.width-750)/2 - 10;
It is always setting the left of the menus to this same value, but if the window is resized, the position of the menu headers changes but this does not, so the alignment gets screwed. You need a way to get the current window width, instead of the screen width.
According to quirksmode.org
if (self.innerHeight) // all except Explorer
x = self.innerWidth;
y = self.innerHeight;
else if (document.documentElement && document.documentElement.clientHeight)
// Explorer 6 Strict Mode
x = document.documentElement.clientWidth;
y = document.documentElement.clientHeight;
else if (document.body) // other Explorers
x = document.body.clientWidth;
y = document.body.clientHeight;
Gets you the dimensions of the window in various browsers.