i think you should leave the original settings for the graphic card. and clocking has to be made with certain programs and certain way. did u just moved it like core and mem -50 or something like this? otherwise this would already greate artifacts on your screen. reset ur default settings. and try some other graphic card drivers for ur graphic card, it might work better than the newer ones for the new card. but not for old series, perhaps 8000 +
control also ur DirectX and stuff, perhaps some windows updates and so on