As this issue seems to have been resolved with new drivers I'll mark this thread as solved now.
I no longer have my 6950, but I had this issue quite often and here's my information if it helps:
Windows 8.1 64-bit. It was clean-installed
Biostar TZ68A+ MB
Intel I5-2500
AMD HD6950 2GB. Catalyst 13.11 beta
16GB DDR3 1333
As mentioned in the OP it seemed to happen during (or around) map change. All the sudden system would be unresponsive until reboot, with all 16 gigs of my memory used by non-paged pool.
I had tried many things to fix it, including disabling the NDU driver and updating realtek drivers and win7 compatibility mode, none of them appeared to help.
I recently sold my 6950 and upgraded to a 280x though. This was a few weeks ago, and I haven't seen a single memory leak issue since. So it doesn't appear to effect the newer GCN amd cards.
