I am not entirely sure this is a bug, but you can have a look at it
Mod Pack: Direwolf20
Mod & Version: Minecraft 1.6.4 Pack 1.0.0+
Whats the bug? What happens is that the game freezes for a second, continues, freezes again, and keeps going
Can it be repeated? Every time I open up any normally generated world with Biomes O' Plenty gen on it happens
Known Fix: I put every setting on low and it keeps happening no matter what
I know this isn't a problem with my computer because this has never happened before with any packs prior to moving to MC 1.6.4
Ah.. But it might be a problem with your system. Most of the 1.6.4 packs are very memory/system intensive. I would suggest you try the FTBLite pack just to see if you experience the same issue. If you don't then you system isn't meeting the needs of the new packs (to slow, not enough RAM, etc.).