Check which screensaver Unity is trying to use, on 11.04 it should just be a blank screen, but if it's trying to use something else it could be locking up the driver when it kicks in.
This is a fresh 11.04 install, fully updated. I've never logged in on the gdm, only on terminal.
Have you tried logging in? I know gdm starts automatically, but I'm not sure if just having gdm running qualifies as having a sufficient X session running. My machine auto logs in my user through gdm, so I have never tried this.
Are you setting fan speed and checking temps?
Both commands work fine. Temps have never been above 90 and usually in the low 80s. When I start the miner, the loads on the GPU are 0%. I have tried this on both phoenix 1.48 (using either poclbm or phatk) and poclbm, result is the same. When the miner hangs, the machine works fine and CPU shows minimal load. When it is running, I've gotten about 810 M/hash out of this setup.
You could try ramping up the GPU fan to 80% and bring the temps down and see if that helps with stability. Yes, they will be noisy. Also consider adding an extra 120mm fan pointed at the input of the card.
What power supply are you using and what is it plugged into? You should probably have a minimum 650 watts for this machine.
I don't think it's a power supply issue. Today, I unplugged the 4830 but had the same issue with a single 5970. I've had the problem with the box plugged in at home and at work.
That power supply should be sufficient. I have run 3 x 5850's on it without a problem.
5970 and 4830 really are kind of an odd combination. With moderate overclocking and good cooling you can get 800-825Mh on just the 5970, I didn't find any specifics, but it looks like the 4830 would only run around 100-125 Mh.