For me at least, after a lot of fiddling, I found a solution to the rate hoping problem.
Im Using Linuxcoin 0.2a with a machine that has 2x5830 ( technically 3 machines, but all are running linux coin with identical hardware )
The problem is the aggression setting on the first card. While you lose a touch of speed off the first card, the second becomes rock steady.
Using pheonix I do something like:
Card 1 ( primary ):
DEVICE=0 AGGRESSION=7 FASTLOOPS=true
Card 2 ( primary ):
DEVICE=1 AGGRESSION=11 FASTLOOPS=false
Make sure you add in all your other standard options
This gives me 298mh/s on card one and 301 on card2 ... both rock steady and shares submitted / accepted averages out to be almost exactly the same ( indicating similar hashing rates With identical overclocking settings ( 1010mhz, 300mhz ram, 1.125v ))