Hashrate has been around 730~750 GH/s quite consistently. Either fried chips or some underclocking..

>Has anyone noticed the mining income became 0.1 per half an hour? The hashing rate was the same as before. How come? They put online another 800 GH without joining the team?

If you look at the transaction you'll see a total BTC25 was distributed to a number of addresses. It was just a lucky block.