Vista - Audio crash during video call

I use an hp pavilion laptop with built in cam and mic. I conduct video chat
with various friends using WLM quite often. Sessions last for at least 1/2
hour with no problems, so I know my laptop is running properly.
Whenever I connect with my Dad, we have problems. His video coming to me is
laggy and very jerky. I see his lips move and then I hear his voice. Our
voice connection lasts no longer than
4-5 minutes before it drops off completely. My dad is using a MS Lifecam
3000 (I think).
If we use Teamspeak (voice only) we have good long chats
with no drop off, so I'm pretty sure his audio drivers are good.
Any suggestions as to what might be causing this??

Thanks