Why does my iPhone 4 keep on disconnecting while using Personal Hotspot?

I'm using an iPhone 4 with iOS 4.3.3. My wifi's speed at home is 30kb/s at max. On the iPhone it's usually 300kb/s, which is amazing. But after few mintues of me connecting the iPhone via usb to my laptop activating a personal hotspot, the internet stops working and in the network settings it says "No internet access". And the iPhone's internet won't work till about 5 minutes, why? How can I solve this? I tried using all bluetooth, wifi and usb connections.