Unable to synchronize colors on dual monitors

I've been trying to get the color between two monitors to be synchronized or at least close to the same for almost one year with no success.

My current system configuration:
- Dell Precision T3500
- Intel Xeon W3520 @ 2.67GHz / 2.67 GHz
- Quardro 4000 video card
- 12 GB RAM
- Windows 7 64-bit
- Current on Service Packs and Patches
- (2) Dell U2410 Widescreen Monitors (1920x1200)
- i1 Display Pro Color Calibration

Problem:
- one monitor looks color correct; second monitor looks more red

Troubleshooting path to current situation:
1. upgraded color calibration tool from i1 Display 2 to i1 Display Pro
    Results: no change
2. replaced Dell 2405 and Dell 2408 monitors (both 1920 x 1200 resolution) with U2410s
    Results: no change
3. replaced Nvidia Quadro FX1800 video card with Quadro 4000
    Results: no change

I'm looking for any suggestions, as I'm running out of options.

Thanks in advance.