So I had this problem on Haven and Hearth 1 too, where it's just unplayable on my machine. For the first few seconds it's fine and runs extremely smooth but then it starts freezing for about 1-2 seconds every 1-3 seconds which is extremely annoying and makes the game unplayable.
I don't think it's my machine because I can run other things fine. I have a Dell XPS 8300, i5 processor, Windows 7 & 6GB RAM. I've never had a problem playing other games comfortably so I think it's something to do with Java & memory leak or something (please excuse my computer illiteracy).
I have the newest version of 64 bit Java and I've tried it on 32 bit as well and I get the same problems... I tried using other clients too instead of the default one and I get the exact same problem... It's not a frame rate or graphics problem, it's just incredibly consistently freeze-y.
Any help would be much appreciated! I already tried some stuff to do with Java like changing the -xms or whatever from 512 to 1024 and stuff like that but nothing is working.

EDIT:
my specs
http://puu.sh/k8AcM/c9147da58c.png