what i learned was that you should never use intensity 20 on a radeon 7990 ....
im running with these settings at 1400 kHs with one card:

setx GPU_MAX_ALLOC_PERCENT 100
setx GPU_USE_SYNC_OBJECTS 1
cgminer --scrypt -I 13 -w 256 -g 2 --thread-concurrency 8192 -o.....

if you look at the hardware comp. on the litecoin wiki you see that this is the way to go on these cards
maybe thats helps your problem...