This is a problem that just began to occur not more than 2 months ago. I've been using Skype for over a year now and I haven't changed anything much over the course of this year, so I'm baffled as to what caused Skype to suddenly be such a big problem for me. I haven't confirmed that Skype is the root of the problem, but I've narrowed down the causes of my constantly dropping internet connection and Skype seems to be the most probable reason. When I completely quit Skype on my Mac my internet hardly disconnects, but the moment I turn it on, even if I'm not actually chatting or anything, my internet will begin to disconnect in intervals of 5-10 minutes. Recently it's gotten so bad that I spend over an hour trying to send an email.
I read in the Skype forums that you could turn of the upnp but that's for Windows. As a Mac user I'm unsure how to solve this problem and was hoping someone could help.
Ugh... I'm also having this problem, and it's almost a year after the OP.
Your issue might be related to how your router/modem handles uPnP or an issue with your available upload/download bandwidth. If you have a Mac and cannot disable uPnP, you can generally find a "uPnP " option in your router and disable it there. So even if Skype tries to use uPnP, your router won't respond by opening connections. Generally speaking loss of Internet is the inability of a modem/router to handle the amount of connections Skype initiates. A power cycle of your Internet equipment might help, but a firmware or hardware change might be required (modem/router/wired/wireless device) to maintain a stable connection if not due to line conditions in your area.