The screen resolution on my imac changes when I use mirror in system preferences monitor

When I turn on my projector before lion it would just project everything from my screen at it's resolution without having to make any settings. But now with lion I have purchased a VGA to mini cable to first connect, but when I turn on the projector I see the desktop image only so it is working but nothing el;se shows.To make it work I have to go into system preference folder and into monitor and click on mirror. What happens now is the screen resolution changes both on the imac and transfered through the projector. Going back into systems preference and try to change the screen resolution nothing changes. I have heard that I may have to download firmware for the cable????, or is there a patch for lion?????. Go back to the old imac with snow leapord and it work fine????. Please can somebody help.

ken hewitt