There can be many reasons. there were times when updating the GFX drivers caused lag and reverting them to an older version fixed them. There are also chances of resource overuse and the system is running low on free resources that can produce it.Seeing as he didnt have this before i doubt he downgraded his system in the meantime
Any specific reason why you post in a 4 weeks old thread? Which is (if you can't calculate) almost a whole month. He clearly mentioned it works again.i got a soultion...
exit (Open Beta) 1.3
go download 1.2.3
and enjoy your low life gaming experince on that game.
Done.