Hi whisky,
it seems we have nearly the same problem...
Bought an "old" PC for nothing with an GTX380 early this year, played WOWs for a while and since march i got the same error message like you.
I have two possible reasons:
1st. Temperature... may be I got a lightly broken mainboard, beacuse when the problem starteted it seemed like it works when PC is warm; thats not really the solution.
2nd. DirectX... check out your DirectX version... may be your GC cant work with your installed version.
I still got the prob and no solution; therefore cant play WOWs
Hope you can play... at least with 10fps...
The error message I get:
MF_ASSERT_DEV_ FILED: BW::Render::success(status)
D:\buildAgent\checkoutdir\client\source\lib\render_context.cpp(3659)