I use iFreeMem, a little utility that tracks free memory, left over from before I got my newer iMac with plenty of memory (a 2.93ghz i7 with 16 gigs).
Recently I'd noticed a few times when things would get very pokey, and I looked at iFreeMem and it showed almost all free memory gone. I looked in Activity Monitor and something called Adressbooksourcesync is indeed using up tons of memory, all available memory, in fact. I quit the process in Activity Monitor, and everything works fine. It seems to happens mostly, or only, when I have the mac Mail program up? Not sure.
I'm not sure where this is coming from, what it is, or how to keep it from coming back?