I use cgminer-2.3.6 on my HD 5870 and received 460-462 Mh/s.
In the new version cgminer-2.4.0 was just 418 Mh/s.
Sys. Win7x64 + OpenCl from AMD Catalyst 12.5 Beta http://www.ngohq.com/home.php?page=Files&go=giveme&dwn_id=1624

Maybe, you should pay attention to the new drivers and optimize a new version of CGMINER