hi,

i had the same issues with my 6870's

try going to start -> run -> cmd.exe and  typing the following (should say value saved)

setx GPU_MAX_ALLOC_PERCENT 100
setx GPU_USE_SYNC_OBJECTS 1


solved all my issues with what your getting right now

if still a few hardware errors are thrown up, try adjusting intensity manually between 10-19 and seeing if theres a sweet spot (mine was around 17)

best of luck