Flickering screen

i have a dell latitude d250. windows xp pro.

whenever i play any video it works perfectly but when I stop that video the screen starts to flicker unless i play any video again then everything goes to normal or the other solution is to put the laptop on the ac adapter, or use windows vista instead of xp and the problem disappears. this problem only happens when I am working my pc on my battery.

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any one got any idea why this is happening, and what is the solution, without using vista instead of xp????