There might be several reasons why your computer freezes.
(1)There might be a Virus in your computer system. Have a good updated virus cleaner/remover and make a thorough scan. It is most important.
(2) This reason is highly likely in your case. Your internet speed is not enough, When you are doing communication over internet that involves video, the internet bandwidth is utilized in maximum. so there for you internet might not bother you when browsing text and still pics filled page. One easy way to trouble shoot this is to go to a famous video site www.youtube.com
and open any video and observe it if the video breaks between playing at least once or twice you do not have internet speed (technically internet bandwidth) for video with any web cam.
(3)There is no problem with your web cam but if your system clears the above test try to reinstall the web cam driver in the computer.
(4)If your OS is Windows. It has a very good feature called "Task Manager". Learn to use it it tells you the consumption of your CPU, RAM (Memory) and Network usage. So you can isolate the problem. In your case the network (Internet connection) will show you high peaks. To open Task Manager right click on the empty area of task bar at the bottom of your screen and select Task manager from the pop up menu. Task manager is very easy and useful and easy to learn.
Contact you Internet Provider (ISP) to get Internet speed increments. May be select a package that suits you.