I am experiencing something very similar. I upgraded to Karmic from Jaunty, which had no problems. Now when I first boot up everything connects just fine and I can use the wireless internet for a short while. Then for some unknown reason a disconnect happens. Network Manager tries to reconnect but is unsuccessful. I am able to get it to reconnect by disabling the network (un-checking the box) and re-enabling it (re-checking the box) a short time later.
I am experiencing something very similar. I upgraded to Karmic from Jaunty, which had no problems. Now when I first boot up everything connects just fine and I can use the wireless internet for a short while. Then for some unknown reason a disconnect happens. Network Manager tries to reconnect but is unsuccessful. I am able to get it to reconnect by disabling the network (un-checking the box) and re-enabling it (re-checking the box) a short time later.
Here is my forum post that has more info: http:// ubuntuforums. org/showthread. php?t=1304554
As of now I am using wicd as it is much more stable. I will gladly switch back to network manager if more info is needed.