Thanks jywarren! I will follow with interest. In the meantime, here are my answers for mips' questions:
Graphics card: GeForce GTX 460
What gfx card and driver are you using?
Driver is the "NVIDIA accelerated graphics driver (version current)", recommended by the Additional Drivers tool.
Make: UMC, Model: UMC-Q19L04L01G-788
What make & model monitor are you using?
The refresh rate, according the NVIDIA X Server Settings tool is set to "Auto". The other options are 60 Hz and 75 Hz.
What refresh rates do you have specified?
What resolution are you running at?
Thanks for your help!