Is there something along the lines of an 'automatic scan adjust', DDNI? My Viewsonic monitor terms its adjustment Auto Image Adjust and one company's TVs call theirs JustScan or something similar. My Samsung TV has the same type of adjustment in its 'PC' menu, too. I haven't used an Amiga with VGA (this Samsung is a picky blighter, anyway!) but thought it was an avenue worth mentioning, as it could have been hard to see what you were looking for if you didn't know of the sort of names these things are given.
Anyway, it's just a shot in the dark. I'm sure you've scoured the whole of those menus that you mentioned, eh? As you got that close, I had to add the suggestion and have still got my fingers crossed for you!