Originally posted by CardiffModderzz
I've tried both av and HDMI cables and does same, I got told it could be GPU but not to sure
The only way to find out at this point would be to continue to dick with it..You have ruled out a few things with what you have already done.
Personally, I would probably try another known working hdd, you never know, before I decided to lightly reflow the GPU(RSX)..You already changed thermal paste, the console was running fine so that should have helped quite a bit. You could also try bending the clamps to the motherboard to assure a good connection between the heatsink and the chip.
If none of that helped, I would think you have a bad flash or something dramatic..