Out of curiosity - how did you get the 5850 to 391MH/s? What's the voltage and GPU frequencies? And what software do you use and how many watts does it use?
I've found mine to be quite stable when undervoltaged to 0.975-0.987 and GPU at 775-785MHz (which results in 305-308MH/s). On the good side - power usage is pretty decent - 675W with 5 of them mining.
Voltage : 1.199v
gpu : 950
mem : 350 (this is the "sweet spot" higher or lower will not give as much mh/s)
It's a sapphire and I use Trixx.
I just lowered it a bit at 381mh/s with a voltage of 1.162 and gpu at 920 to have less fan noise. I have instruments to measure the power usage but i'm at work so I don't care

This can be easily done at stock voltage mem@150 core@840. You only need to replace cooling paste with Arctic silver. Usage is about 180... And I have tested your settings. I think you need batter kill-a-watt. I can't make it run with the usage that you reported last time... It uses about 120W+. If I remember correct you reported 85W.
That won't give you 391mh/s at all more like 330.