Ill have to agree with the lads on this one, your RSX Substrate its about to fail, now the question is when is going to fail?
Usually systems that get the GLOD from a faulty GPU don't come back to life, unless you reflow the RSX or replace it, due to the substrate failure within the GPU itself,this is why i always recommend deliding the RSX and keep it as cool as possible, its more important than the CELL B/E itself.
GPU artifacts come and go, usually it can go away, but eventually it will come back, one little test you can do, its pick a heat-gun, just for diagnostics, and heat the RSX Chip at approximately 150°Cs for a 5 min mark, this will make sure the substrate makes proper contact with the Chip Die, thus eliminating any artifacts present, but eventually it will come back, and if it does then the RSX is the problem, you'll need to find a 90nm donor to replace the faulty one, ive seen the RSX artifacting on many Backwards Compatible machines, some more heavy artifacting than others, such as corrupted XMB wave, incorrect colour palet, flickering screen and many others,but you can recheck the Tantalums on the RSX area again,but i don't think you've done a bad solder job to detach that easily.
You can also Delid the RSX and replace the thermal paste underneath, this also can Fix GPU artifacting if is a light artifacting due to thermal stress, which upon replacement it should go away,but its not guaranteed.
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