well maybe this is due to the fact that the part of circuitry used for internal display are used more often, so the increased electrical current causes more material loss in that section of GPU bump material then part responsible for the external monitor.
The best way is to get the post warranty warranty and get that fixed under the post warranty (it would be cheaper than sourcing your own motherboard), as suggested here:
FYI, the GPU is integrated on the motherboard, swapping motherboard would not wipe the stuff off the hdd, unless you have some sort of special security measure that automatically wipes hdd when it senses that it is ran in a different machine. But it may involve reactivating the Vista or Windows 7 OS if you have that installed.
May this year, be the year of 'DO'!
I am a volunteer, and not a paid staff of Lenovo or Microsoft