cant play mc pc 1.6.1 or 1.6.2

I have had the problem that when I open mc pc 1.6.1 and 1.6.2 I can play one player mode and no problem but when I go to play multiplayer it shuts down my internet and it wont come back up. I had 2 servers that worked for one refresh capture the flag and mc-zones. my mc name is Turtlebacca and I will do any thing to get the problem fixed pls help. I can play 1.5.2 though.