Monitor on Win 98

Screen resolution and color depth (regardless of the operating system) are a function of the capabilities of the monitor and the graphics card installed.

Most of the time, if your settings are incompatible with your hardware it is immediately evident and doesn't appear over time.

Since your problem appears to be intermittent (it changes over time), I would suspect that there is a high power source of electrical interferance near the monitors affected. Try plugging the monitor into a computer in a different location and see if the same problem occurs. If not, this will rule out the monitor. Next, try moving the computer to a different location. Again, if the problem doesn't occur, you can now rule out the computer. That leaves the location. Large electric transformers, electrical personal heaters, high power electrical lines, large electrical motors, etc. can all generate electrical fields sufficient to mess up a monitor.

I agree with Tim that you have an inherent power supply issue to deal with as a test increase your resolution to max on half of the monitors and see what happens, these are fair machines and they can handle higher res rates. Check your network cabletoo, is it running along heaters, air-cond, etc; phone Dell and talk to their service people...:) ....hope this helps