hi, i've been having the same problem on one pc but not on the other, on my home network.
in fact had a similar problem on my ex's pc, and the two problem pc's both had the same anti-spyware installed - ClamAV for Windows.
Which is excellent software, but I think it could be the cause, for me. I've just disabled it's realtime scanning and its been ok for about 10 minutes, which is some going for this pc
So I think it could be your anti-spyware program - worth a look?
just to confirm, just been chatting for over half an hour after i made that change. was definitely the problem.