You could try edit your /etc/X11/X86FConfig-4 file. This might not work.
Go to Section "Screen" --->modes:
Try add and arrage your resolution mode with desired mode first. Ex:

Modes "1280x1024" "1024x768" "800x600" "640x480" or you could remove "640x480" if you want to.

Edit this line to other subsection too.

Don't blame me this won't work. If you cannot start X after done this, restore you screen mode using mc.