BenQ Joybook 5000G display abnormal

OK! When the PC boots, the maker's logo is distorted & the POST has boxes all over the screen. Occasionally, it would display words clearly but are jumbled & gibberish to read. When it boots into Windows later, the display would be fine for a moment & then a somewhat 'shadow mask' kind of lines started appearing, this problem would also appear instantly at other times. The display gets worst thereafter. One particular thing I'd noticed about it during surfing the web was the area inside the browser window appear fine, though rarely. It got the worst when I scrolled pages left to right or vice versa. I asked a few repair technicians and they point to the card is in fault, though not all couls warrant a recovery solution except to replace the entire board. Actually, I'm wondering whether graphics chip's IC do corrode? I heard from a friendly technician who had never dealt with BenQ laptops before, that it is almost impossible to see a corrosion physically on the graphics chip. It's even almost impossible to point the problem right away. Any idea, solution or experiences to share?