USB install CGMiner

Has anybody encountered this and hopefully solved it?

I have a Miner with 6 x 5850‘s. Previously I had 5 x 5850‘s with the system booting from a HDD and no problems. Now after adding the 6th 5850 I cannot boot from the HDD (old and crappy) so I am trying to use a USB stick.
When I use a 16Gb “HP” branded USB stick with clean full install of Xubuntu 11.04 64B, SDK 2.4, CGMiner 2.4.1 and 11.6 drivers the system starts and runs perfectly at full performance for about 15-20 minutes. It then starts slowing down, ie sort of “choking” down and if left alone will finally go down to about 50 Mhs per GPU after about 12 hours.
The install is straight from the 11.04 ISO CD with no additional packages installed apart from SSH, Screen, Curl, SDK, AMD Drivers and CGMiner all as per Kano’s instructions.
I have remade this 16Gb USB stick twice and still have the same result each time ie the slowing down of the GPU's. This is exactly the same install and configuration that was installed on the previous HDD which ran without problems for months.
If I use a USB stick made from the “Start Up Disc” creator in Xubuntu 11.04 via Kano’s instructions The miner runs perfectly using the same config and NOT Choking (however I cannot get SSH and Screen to work on that install.)
I have looked at syslog and kern.log. I dont understand it, but the file sizes are 566,820 & 650,596 on the 3rd startup of that install. Is this correct
My guess is that there is some issue in how the full Xubuntu 11.04 64Bit install writes or communicates with the system as opposed to how the Start Up Disc Xubuntu 11.04 64Bit install communicates.

Anybody know?

Specs
Corsair 1200
MSI FXA990-GD80
AMD 2 x 260
2GB ram
6 x 5850 (undervolted)

Edit:- Maybe somebody else will have this problem-- I am not sure but this is what I have guessed. 1:- the 16Gb USB is painfully slow seems bigger they get, slower they access. 2:- I had not disabled the Screensaver (as per the CGMiner readme) The screensaver was kicking, for some reason causing 100% on 1 CPU core after about 10 minutes.

Those two things combined just choked the USB Stick. Same install on a USB remote HDD worked fine.