GUIMiner Stops submitting shares

[Stizuner]

Hello, I have a machine with two 5870s and it used to work fine but now it will stop submitting shares.

Visually in guiminer it shows that its still working - but i'll get a notification from the pool that a miner is idle and when i look at the submitted last share time in guiminer it will say xxx time.

Any ideas what could be causing this?

Thanks