Any way to force on/off the fan on an nVidia Quadro FX 3500

Are you using DisplayPort to DVI converters? They need to be active adapters, or you can't run more than two simultaneously. This is true on 99% of hardware.

If you're using active adapters (or native DisplayPort) then I have no clue.

I had this problem for ages. I looked online and finally found ONE post that made a suggestion that actually worked.

http://forum.notebookreview.com/dell-xps-studio-xps/570946-xps-15-l502x-constant-fan-off-8.html

The solution to the on/off/on/off problem is to go to the "Power Options" and select the "Power saver" radio button option. You can then change the internal contents of the "Power saver" options to whatever you wish, but it must be selected.

I don't know why it makes a difference; it may be the combination of several items... may be the option itself. In either case, My fan turns on VERY infrequently now even when watching a movie. I suggest you set the option to "Power saver" and see test things out. From then, you can tweak your heart's content.

Hope this helps.