Shoutbox

The problem here is most likely an issue of bandwidth. More specifically, your actual Internet connection will be struggling with the load a webcam and audio put on it. This can be due to a number of factors, including (since you're on ADSL) the distance from the telephone exchange.
Since ADSL goes over normal copper phone wires, the longer they are, the more the signal degrades, and the more likely it is to get corrupted. This affects both the ability to get the higher speeds, and to more consistently obtain the speeds your line is capable of.

I know this from experience: when we tried to get ADLS in the house, we were told that we were too far from the exchange to get reliable 8Mb, and we could only really get 2Mb. And that slows down sometimes.