15 in. E152FPb Flat Panel

I just purchased a Dell Dimension 4600.  I had a 4100 which was still good so I puchased a  15 in. flat panel monitor to use with the 4100

model #  E152FPb.   At times I get horizontal silver lines on this monitor that look like small transisters running across the screen.  This has happened three times in one week . I reboot the computer and it works ok.  Now I am testing the monitor. I have moved the 15" to my new system (4600) and am using my older monitor an 17" flat panel back with the 4100.   Does anyone know what could be causing this problem?