Hashrate showing much lower on pool than in cgminer on both ghash.io and slush

Hi,

I've been mining on ghash.io with my mining rig that comprises a Raspberry Pi, powered USB hub (2000mA) and 2 antminer U2 ASICs.
Since I had it set up, it has been mining at 3-4Ghash/s consistently. That hashrate has been reflected in my ghash.io page. However, I noticed that suddenly, the hashrate shown on ghash.io for my miner has dropped to a significantly lower number and stayed there: cgminer still reporting 4.something GHash/s, but the stats on the webpage showing around 350 MHash/s. I wondered if it was a problem with ghash.io so I decided to try slush's pool.

So having registered and set up my worker and reconfigured my first pool in cgminer to be slush's pool, I restarted my cgminer. It connects fine to slush and the worker shows up fine, but it is showing a hashrate in the 600's MHash/s, even though my cgminer software is still showing around 4 Ghash/s.

Anybody any ideas about what is going on ?

Is there some kind of network tuning I need to do ??

Cheers,

Dave