NVidia Quadro FX 3800--Blank Second Monitor

One thing I don't have much experience with is dual monitor configurations with newer graphics cards, so I'm hoping I'm just really inexperienced and someone out there will know this one cold. We've got a Dell Precision T7500 equipped with an nVidia Quadro FX 3800 graphics card (one DVI and two DP out), running Win7 Ultimate x64, and we're trying to connect two monitors to it (one Dell 2410 and one Dell 2420), both at 1920x1200. Our cabling selection is sparse, and I wonder if this is the problem. Here's what we've tried:

Both monitors work individually, using a DVI-to-VGA adapter on the DVI out. Neither works as a single monitor with either of the DP outs, using a DP-to-VGA adapter (I believe a passive adapter, since there is no USB or power connector). If I use both, connecting one to the DVI and the other to a DP, then the monitor attached to the DVI always works. The monitor attached to the DP will occasionally flicker, but no sustained visual.

I have adjusted properties to change primary monitor and other various properties in both Windows and the nVidia control panel, and neither seem to be effective at getting the hardware to work. Although Windows recognizes both monitors when connected, it will only display to the one connected to the DVI out. The one exception was early in the testing process, when Windows recognized the 2410 as a generic PnP monitor. I was able to get it to work as the secondary display in 1280x1024 with the generic driver. Thinking that I had found the solution, I downloaded and installed the driver for the Dell 2410 and both displays worked at that point, in full 1920x1200 definition. Then, in an effort to adjust the primary display, I hit the Detect button in the Display Resolution window and lost the second display altogether, never to return.  

Further research tells me that the card offers up to 3 digital displays (only two of which can be used at a time) and one analog display. I'm not sure if this is affected by the adapters we're using, or if the adapters themselves are causing the problem, or if something else is in the mix that I haven't even considered.

Does anyone have any ideas about this one? I'd appreciate the help.

Thanks...