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About the GPU repair......who did the repair? Was it a Apple certified repair technician and shop?
In the past Apple Hardware was all proprietary......but now.....not so much. They use same CPU's and GPU's as the rest of the computing world.
So, your having issues with a product that has known RECALLED components, and now that the recalled component has been replaced.....the issue is not yet resolved, or more issues have risen.
Honestly.....i dont think your going to get any help without the tech being able to take this thing apart and swap out parts for KNOWN good parts (components) at a tech bench.
But i will OPT-out to allow others to take a stab at it.
I have never heard of anyone doing only a GPU repair as that is an integral part of the motherboard on a MacBook but anyway.... can you give me the serial number? You cannot run anything higher than Lion on a MacBook 2007 see here: https://support.apple.com/en-us/HT202575