My sapphire 7950 on a computer with 4gb ddr2 cannot go past -i 13 without getting hardware errors, I've tried thread-concurrencies for hours. Currently it's only doing 160kh/s

My XFX 7950 is fine and pulling about 200kh/s   --thread-concurrency 16384 -i 19

I've yet to get a 5870 to run at any -i 11 without errors