Dual Monitor - resolution drop

I have recently set up a dual monitor system on my PC. Looks great and works fine most of the time. Sometimes, however, especially when my computer is working hard (i.e. when i can hear the CPU humming) or often when i access 'programs' on the Start Menu, the screen on my Primary Display goes black for a second or two, and when it returns the resolution has dropped a few notches. I keep both monitors on their highest possible resolution. The primary usually drops from 1680 x 1050 to 1440 x 900. I even purchased a new graphics card in order to set this system up.
My secondary (smaller) monitor remains unchanged.

Does anyone know what could be causing this???

some specs:
OS: Windows XP - Home (2002)
PC: Dell Dimension 1100 - DE051
Processor: Intel Pentium 4 - 2.8 GHz
Primary: LG 22'' Flatron Wide (L226W)
Secondary: Dell 17" ??
Graphics Card: NVIDIA GeForce FX 5200