it may happen the same thing like what happen to my card.Your card uses one timingset when Geforce and another when Quadro.
The timings that are used when quadro are too tight and have to be modified whith the ones your card uses when Geforce.
Do one test, with the quadro rom, open nibitor and in the main tab go to Tools/Read Bios/Select device, select your card, hit ok and again go to Tools/Read Bios/Read into Nibitor.Then go to Bootup Clocks(main page, down),open and hit Autoselect, you'll get "your timingset id no..."and check Core/Shader/Memory to be 400/800/400.Remember the timingset which your card uses when quadro, then reflash the original rom, install the forceware drivers and open Nibitor again.Do the same thing in Bootup Clocks and see if the card uses another timingset.If it uses another one(for example quadro uses timingset 2 and geforce timingset 1), here is the key - you'll have to modify the timingset which quadro uses, accordingly to the timingset which geforce uses.
Open 2 instances of Nibitor, left, one with geforce.rom, right with quadro.rom.secet in both Timings/the timigset every card uses and modify the detailed timings of quadro's rom with these of geforce rom and when finish, hit ok.(do not forget to select the timingset which your card uses when geforce and when quadro)
when you'll finish to modify, and before saving the rom with ajustments, close geforce's Nibitor instance and in the quadro instance go to Adv. Info, hit rescan bios, you'll see that the green dot (Integrity) will become yellow.You'll need to adjust the new calculated checksum of the bios.
Go to Tools/Hex View and in the hex editor that will open on the right, scroll down on the last line and look at the last two digits, they corespond with the actual checksum, but you'll have to modify them with the new calculated checksum.Look here:
Then go to File/Save Bios.Reflash the new rom with timing modifications, reinstall quadro/geforce drivers and download OCCT which is a stability testing program.Extract it and run first GPU:OCCT with "error check" ticked for 15 min and look for errors and videocard temp.If it fails and you get errors, stop it and run GPU:MEMTEST pass:10 and again, look for errors.If your card runs at stock clocks and you still get errors, then you'll have maybe to downclock memory(for unknown reasons, this what's happen to my 8800GT@Quadro FX 3700).
Aquamac was so kind and made a custom rom after my original one and still have this problem.Maybe I'll have to write to Mavke at http://www.mvktech.net to beg him some help to make Nibitor use the same timingset when OS recognize the ex Geforce as Quadro.
WOW, huge post, I hope this will help your problem.