Skype stops responding after a while

"If you have pulseaudio on your machine, try configuring Skype to use pulse as per the instructions on the pulseaudio wiki. If that doesn't work, look through the Troubleshooting Skype of the Ubuntu Community documentation.

Here's a post on the skype forum that might be of interest: Skype hang when calling or going to option->sound

I have experienced something similar. The next time Skype stops responding, open up the Options->Sound Devices page and see if you're able to reconfigure the devices. I think this is a common problem with Skype on linux."