Alas... poor HS transplant. That is the same way my GF4 TI4600 met it's demise. Except I accidently gouged into the PCB with a screwdriver trying to pry the damn HS off
Ive had two cards like what you are describing, a GF FX5900U and a 9700Pro that would boot just fine, but had wierld lines of random characters, or just tiles of garbled video. (The 9700 pro half the screen was crystal clear, the other was garbled to hell. There is no fix to my knowledge I also tried for hours on end to fix these cards to no avail. This type of damage is generally caused by a static discharge when handling them, or sometimes even a Piece of crap power supply that gives the card a random extra jolt.