Actually, it's.. well I'm not really sure what to call it but this took me an Age to figure out.
The problem is - or at least was when my OP2 got stuck in fullscreen 640x480 mode - the game's detected you're in a colour setting it doesn't like and switched to DirectDraw mode, which only works in 640x480 resolution.
Just mess about with you're colour settings ( High Colour, True Colour, etc.. ) until you find something it likes. 'Hope you're having the same problem I did and this fixes it!