Hello,
if already the bios sreen is garbled and if a cool system resolves the problem, it seems you have
a faulty Nvida GPU, it´s a G64 Chip in your gfx card and Nvdia´s G64 cannot cope
with normal heat. This problem is known by Nvidia.
I would call Lenovo and tell them your problem.
moderator note: check out Marks information here, too.
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I´m a volunteer here using ThinkPad 13, X1 Carbon 2nd Gen., Yoga 900, ThinkPad Yoga, T430s, IdeaCentre B540.
TIP: If your computer runs satisfactorily now, it may not be necessary to update the system.
My warranty expired in July. 
and I started facing this issue in August.
The customer rep. I spoke with, put me on hold while he said he will check if I was correct about the bad Nvidia GPU issue. He came back after 10mins and blatantly informed me that there is NO SUCH issue, and that I will have to foot the bill to get this repaired !
