High but unstable hashrate - what's the cause?

My rig:
Gigabyte EP31-based MB, Intel E5200 (dualcore Pentium downclocked @ 1.5GHz, CPU load is about 10% per core), 1GB RAM, Windows XP SP3; PSU 950W;
3 * Sapphire 5870 OCed to 950/300 on PCI-e 1x risers (temperature is 60-70 C);
guiminer/phoenix/phatk; Catalyst 11.6 with APP SDK 2.4.

Each GPU goes upto 460MH/s while mining standalone.
If I setup 3 miners - 1 on each 5870 - I get only 0.9 GH/s - 1st GPU load is 99%, 2nd and 3rd - about 60-70%.
To maximize hashrate I've setup 2 miners on 1st GPU and 4 miners on 2nd and 3rd each. Now I've got 99% load on each of 3 GPUs and hashrate as big as 1400 - see picture attached. But hashrate is highly unstable - it hops from 1100 upto 1500.

https://lh5.googleusercontent.com/-TqJg0J3uirU/ThRWG5fZFDI/AAAAAAAABH8/JLChGHFiEOE/hashrate.jpg

Is there any problem with my setup and what can I do to stabilize hashrate at 3 * 450 = ~1300 MH/s? Or would it be better to do nothing?

Topic: High but unstable hashrate - what's the cause? (Read 747 times)