Hypertransport Sync Flood Error

Okay first off, I know there's a lot of forums and threads about this issue, but I just CAN'T seem to find one that has the answer I want.

Anyways! I'm having issues with my brother's rig, here are the specs:

AMD Phenom II X4 955 @ 3.5ghz (17.5x200[if I remembered correctly]) 1.425v
Biostar A880g+ Motherboard
Xfx Radeon HD 6870 (Completely stock frequencies)
G.Skill 1333 Mhz CL9

So basically, we're getting the error whenever we run anything that seems to be CPU/GPU intensive, for example, playing a game(Civ5). Yes, we've tried to put the CPU back down to stock settings, but in the end we still get the error. We got the Radeon HD 6870 on February 22nd, and his rig was working fine until the 13th of this month. So we're both wondering if this is GPU related or if it's a bad motherboard/CPU. Since I know his motherboard isn't the best, we're planning to upgrade next month (hopefully it'll rid us of this dreadful error too).

Thanks in advance for any help :) hopefully this isn't a bad Mobo since I'm wanting to make a HTPC for my mother.

****EDIT**** Forgot to list the PSU, we're using a Corsair TX650w PSU.
Did you make sure to get the latest BIOS? Might fix it.

If not, it can only be mobo/CPU or RAM. I would run Memtest 86+ overnight to make sure the RAM is fin.

unlikley its the CPU since CPUs raley go bad so if its not the RAM I would lean towards the mobo more than anything.
Yeah, all the drivers are up to date, as well as the BIOS (last update was in 2008 unfortunately...)

I'll try running Memtest for sure, since I also figured my CPU wasn't the cause...worked fine in my personal rig. I'll get back to this thread after running memtest. Thanks for the help :)