Egg gave you a good suggestion.
Other thing that you might want to consider is that Firewall to Firewall is not a recomended connection accordint to MSN. Of course, this is usually a likely scenery, so it is %$@
I had that problem all the time when trying to send files to my friends. IT always depended in which PC they were and what type of security it had.
One clear example is, when I use my NAT enabled router, sending files might or might not work, and there is nothing I can do about it, but use YAHOO IM.
In that case, also the web cam of YAHOO frezees a lot (not mine, but the other user) and if I take out the router it doesn't happen.
After over a year fighting with that stuff I gave up, and just used what I could
The newer version thought it is a lot more friendly to NAT than previous. The worst version?? 5 until version 5.1. That even would disconnect every 1 minute, and show me offline even I was chating with friends. They would tell me thru the messenger that I appeared offline. If we didn't keep chating the connection would get lost.