USB Internet Tethering unable to re-establish connection iPhone 4

Hi,
Im sharing my iphone 4s internet connection with an ibook on 10.5.8 via usb. This works fine, but whenever I unplug the usb dock connector and reconnect it after a break, I dont get the connection automatically re-established.
I do get the blue bar on my phones display saying internet tethering but theres no sign of a connection on the ibook. Only after fiddling with the network settings I manage to get connected again.
Any ideas? Many thanks in advance!
Giorgio
iMac Mac OS X (10.6.4) Intel core 2 duo 2.16ghz, 3gb ram, 256 vram xt1600, iPhone 3g, iPod 5th gen.