Appears this has been an issue with nVidia cards and drivers and certain chipsets and motherboards for the last couple of years: http://forums.nvidia.com/index.php?showtopic=153911.
Funny - I never heard about it. If you go though this 28 page thread, you will see people have tried pretty much everything with limited to no success.
I have downloaded GPU-z which has allowed me to monitor the card both at idle and under load. At idle and low graphics usage, my GTS450 sets its PCIe bus to rev. 1.1, 2.5 Gb/s speed. Under load, it sets it to rev 2.0, 5 Gb/s.
I suspect this is related to the m/b power management allocation; GPU-z help says as much.
Problem is no one seems to know if this on-the-fly bus speed adjustment is good or bad. It appears equal to taking a 4 lane highway and reconfiguring it to a two lane highway depending on traffic volume. When you think about it, it doesn't make any sense to do that .............
I might RMA the card and get an ATI one. Appears nVidia's glory days in the graphics card world are very ancient history.