My laptop is a dell xps 1645, with an i7 q720 and 4 gb ram and ati radeon m 4670. Games like fifa 13 used to run perfectly on my laptop. recently the fan failed and it a few days before i realised it... I started noticing the really slow performance in my games, so i had everything changed from the fan, heatsink, ram, cpu, and motherboard fearing damage to them.
The games though are still slow as they were just before the hardware was replaced.
Could you download HWMonitor (http://www.cpuid.com/softwares/hwmonitor.html), open it, start playing the game, wait for it to start lagging, then alt tab back to the HWMonitor, maximize its window, take a screenshot of it, upload it to www.imageshack.us (don't forget to select "do not resize" option!) and post the link to it here?
Anyway, I missed that bit - I'm sorry. Could you tell me whether you didn't forget to reinstall windows when you replaced the motherboard? If you haven't done that, you should, because when installing windows, they kind of tie themselves to the motherboard.
as long as the motherboard is the same chipset normally re-installing is not needed, but it is worth a try, my guess is the GPU is faulty and just overheating, and being integrated a whole mainboard would need to be replaced