HIS 7970 iceq x2

If i don't specify a thread concurrency and only specify shaders of --shaders 2048 cgminer will auto pick the thread concurrency for me. It picks under 8192 and my hast rate is 700 but i get lots of hardware errors.

As soon as i specify a thread concurrency of 8192 i get 600 hash rate but no errors.

Why can't i use under 8192?