I have same issue.  I have a Sapphire 5850 Xtreme:

OS: Windows 7
Miner: POCLBM GUI (any flags will cause the miner to crash)
Drivers: Latest
ATI Stream: 10.4
5850 Model number: Sapphire 5850 Xtreme Edition 1GB

Mine rate: 220 Mh/s @ stock clock, 240 Mh/s @ 775 Mhz, 262 Mh/s @ 850 Mhz

My card is not stable at 900Mhz so I have not tried it yet, I will probably need to adjust the voltage.