Black screen after switching users

I recently installed a HD3450 ATI graphics card. I am already using the latest ATI driver. I’m currently booting the system from GRUB with vga=0x03e6 (since optimal display’s resolution is not available in VESA).
Also have 2 users: A (screen resolution 1900x1200) and B (screen resolution 2048x1152 - optimal resolution for display Samsung Syncmaster 2343nwx).
Every time I switch from one user session to another display turns black, like if no signal was coming out from the graphics card.
Any clues? Thanks!