On your monitor there should be an "image setup" button or similar. This should automatically lock the monitor to the amiga's screen size, position, phase and clock.
This works best if you feed the monitor a checkerboard test pattern to lock onto while it's doing the image setup.
So display this image fullscreen on your amiga
http://www.techmind.org/lcd/phasing.html, and while it is displayed run the automatic image setup option on your monitor and see if it helps.
