Had similar problem with my Albatron GF4200Ti, except video corruption could be seen only in 3D apps and it was minimal. Tried to locate bad memory IC using VMT, but memory addresses, that returned corrupted data, helped me little to nothing, because you need to know EXACT memory ICs organisation in memory BANK(s) for that info to be useful. Managed to pinpoint bad SDRAM by playing with different clock rates and cooling them down one by one, while observing video corruption levels.

But if you get garbled characters/vertical lines while in 2D, then problem could be anywhere: cut traces on PCB, missing resistors, detached or dead memory ICs, detached GPU (contacts responsible for memory address/data/control signal lines), shorting balls under GPU/memory ICs (caused by debris or conductve silver thermal paste), even bad connection with AGP port could be the cause.

If you suspect detached memory ICs or GPU to be the cause, you could use an F-clamp to press ICs hard enough for supposed bad contacts to reform.

Thanks for the reply 😀

So, basically there's no way for a layman to go about it in a scientific way then?

Went through the card another time under the USB microscope, couldn't find any physical damage this time either. It's of course impossible to check for shorts under the different BGA packages, but tried cleaning with contact cleaner and compressed air. Also, the only TIM that's been used is the stock one, and the one I applied when replacing the stock heat sink. Both are ceramic based. The AGP connection should be ok, same symptoms in two different mobos.

Tried your clamp trick - smart one by the way - using a C type welder's clamp and two pieces of rubber sheet glued to the jaws for protection. Couldn't see any difference in artifacting though.

I'll just try reflowing all the vram chips and the GPU, if it won't work I'll just give up. It's not like it's a Voodoo5 or some other rarity 😀

Welder's clamp will hardly exert enough force for BGA to reconnect, also, rubber base would absorb fairly large ammont of force in the form of deformation (unless it's harder than that of car tires). You need to use either F or C clamp with thread and a wooden base (medium hard, like birch) the clamping force should be between 50 and 250 N (5-25 kg) for effects to be noticeable 😀