Well, considering I've had it running for over an hour now I don't really think anything is going to happen. At some point the memory jumped back down to 9%, and although it started going up again I'm guessing it will just jump down again later on.
I tried memory sampling the game, but nothing stood out to the point that I'd say that could cause any trouble.
It would seem that this issue is at least in part not tied to the world, which would eliminate a lot of potential causes. Unfortunately it also leaves me without the slightest clue as to what could be the cause. It does however explain the oddity that the lag didn't persist, as in re-opening the world would "fix" it temporarily.
The only thing which occurs to me which might be a cause (although it shouldn't) is that perhaps the garbage collector isn't doing it's job properly. I am at a loss, although it could be a java issue, so do update to the latest java.
Also one last thing to try with a backed-up world: disable the mods one at a time (or disable sets of them or something) to try to isolate the problem if a mod is causing it.