Experiencing the same problem.
Seems to be connected to the internet connection/router
At home all my computers (Linux Laptop,Win7 x64 PC.Mac Book) have this Problem.
Using my Laptop at University and other places it works fine.
Connection Speed doesnt seem to be the problem, a friend of mine with slower connection doesnt have the problem.
Client Version doesnt matter (Tried different versions of Enders and Native).
Looking through sourcecode but didnt find anything useful yet.
PS: maybe connected to this viewtopic.php?f=4&t=19416