What does the output look like with -D until it has started getting HW errors?
Output from minerdI didn't set --queue, --gpu-threads or --intensity, so these were at default values.
These lines look interesting:
[2011-07-04 01:42:48] GPU 1 found something?
[2011-07-04 01:42:48] No best_g found! Error in OpenCL code?
This only happens with the last few versions of of minerd, an older version (before your "What next..." post) worked fine with both GPUs. Unfortunately I don't know at which exact version it stopped working.