screen flicker

The flicker happens after the computer has been idle for a while,then settles down after I connect to the web with no further flickering.I have a Dell 4100 with the ME operating system.A new video card was installed(ATI Rage 128 Pro 32 bit) after the original card died.The refresh rate is set to optimal in the 800x600 setting.The new card has been working fine untill a few days ago.What can I do?