"-f60" and above can help.
in general, try keep you GPU saturation under ~92%.
about 69x0 specific. similary behaviour 58x0's. its more depend on driver verions, than HW. some was hang, somw was not[usually, drivers w/o kernel-space thing. "atimdagxxx.sys" or something like].
I've never had flash or anything GPU accelerated work, regardless of -f setting (which I believe should be -f 60, the space is important).
And though I've had the problems with my 6990, I never saw anything similar with my 5870 nor 5970