Graphics card red lines

hello.....
i have some problem with my graphics card....its 9800gt....it was crashed months ago n started displaying red lines.....n i never used it again....after few months i tried it again after aplying thermal paste n it worked fine for whole day.....lines were gone....next morning i had to apply thermal paste again to make it work.....but thermal paste dont always work . there may b another reason....i would realy appriciate ur help....(sorry for my english :p)......