Hitachi 21" monitor problem

[mumbles]

I just bought a 2nd hand 21" monitor to use in my mame cabinet. When I try to load some mame games in mamewah it comes up with invalid scan freq on the screen and won't load the game. Does anybody know if this is a problem with mame, mamewah or my monitor.  It happens with Shinobi, Shandow Dancer and others and I wouldn't expect these games to be too technical for the monitor to run.