I had the same problem with a 7950 card (using Catalyst 13.4 and 13.5beta drivers) on scrypt-based currencies. After a day of fiddling with settings and I found the solution: force the vectors to 1 by adding  "  -v 1  " to the command line script. That should fix the HW errors.

My full set of working options are -s 20 --lookup-gap 2 -I 17 -g 1 -v 1 -w 256