How do I set 1680 x 1050 resolution on my widescreen auxiliary monitor?

I've searched many posts on this topic, but could not find an answer to my problem. I'm running a Dell XPS M140 laptop connected to a Gateway FPD2185W TFD LCD monitor.  The Gateway monitor's native resolution is 1680 x 1050.  When I purchased the monitor two years ago and connected it to my Dell lap top, I was able to select the 1680 x 1050 resolution from the Display-Properties-Settings window.  I even recall seeing the Gateway monitor listed in the display options pulldown window.  I was convinced that it was working fine. 

However, recently, I noticed the 1680 x 1050 resolution option missing (the options skip from 1400 x 1050 to 1600 x 1200), and I do not see the Gateway listed.  Rather, I see only "1.  Default Monitor on Mobile Intel(R) 915GM/GMS, 910 GML Express Chipset" and "2.  Plug and Play Monitor on Mobile Intel(R) 915GM/GMS, 910 GML Express Chipset."  The same is true when I look at My Computer-Hardware-Device Manager-Monitors.  I see "Default Monitor" listed 3 times and "Plug and Play Monitor" listed twice, but no Gateway. My understanding is that the Intel 915GM/GMS chipset supports the 1680 x 1050 resolution, and that the chipset or videocard should recognize the Gateway monitor.  I've tried to get help from the Dell online after warranty support line, Gateway chat, and EzTune (the company that makes the software for the Gateway monitor). No solution so far.  The EzTune tech said I needed an updated video adapter driver, but when I went to the Intel website, the one that he suggested was "not validated" for the Dell computer.  The Gateway monitor comes only with a CD for installing the EzTune software, but no drivers.  The Gateway tech told me that the driver for the display is in the laptop and not the display and to contact Dell.  When I spoke with the Dell tech, he did not have an immediate solution because he was not familiar with displays, but suggested that I do a PC system restore back in time to try to recover the old setting.  This did not help either, and I can only go back one month, which may not be far enough. 

Trying to help myself, I poked around trying to find other symptoms.  I did see that under Control Panel - Intel Graphics Media Accelerator that the 1680 x 1050 resolution is NOT listed, which is consistent. I tried using the Wizards for searching for new hardware and getting driver updates, but no success.  The Wizards told me that I have the latest drivers installed. My hunch is that somehow my video adapter or monitor drivers have been upset or lost, why, I don't know. I was able to get the 1680 x 1050 resolution earlier, so this is very puzzling.  Maybe a Windows automatic update did something to my system configuration, but I am only grasping at straws.

I appreciate help from the forum community with either a solution or where to go to find one. 

Thank you.