Open the file:
/etc/X11/XF86config-4
Have a look under:
Section "Device"
Identifier "Card0"
It is is nvidia, it will have a line which says:
Section "Device"
Identifier "Card0"
Driver "nv"
Have a look at the resolution, e.g.
Section "Screen"
Identifier "Screen0"
Device "Card0"
Monitor "Monitor0"
DefaultDepth 24
SubSection "Display"
Depth 24
Modes "1024x768" "800x600"
EndSubSection
SubSection "Display"
Depth 16
Modes "1024x768" "800x600"
EndSubSection
SubSection "Display"
Depth 8
Modes "1024x768" "800x600"
EndSubSection
EndSection
Change the line:
DefaultDepth 24
to
DefaultDepth 16
If it is already 16, then leave it alone. After changing that, restart X windows. If it is still a problem, then you can rule out a graphics problem and change it back.
It can also be a power managment function with the laptop battery, so I can't give you any advice in that area as I don't use laptops.