Give what Largg said a shot, hope it works.
When it comes to connection problems it's always kinda hard to find the reason.
If no one else is having problems and you guys with the problems live anywhere close to each other it could always be a routing problem somewhere between you and the server.
So if that is the case, odds are it's gonna fix itself in a while.
Else you will have to do some trace routing and stuff to find out where the problem is.