I have had this issue as well. Its Video related - I got the "video memory almost used up" from POTC Support, but who knows as their suport is spotty at best... and I don't rate it even that good.
I reviewed the logs myself and along with my testing different vid cards its more of a guess (though I hear more ATI users or low memory cards have more problems than other cards).
I wouldn't be using the crappy card I use now if my good ATI card would work with the gam e- but that card doesn't even allow me to sail (crashes when boarding), and POTC Support hasn't answered that one - three months later...
Good luck though, I hope your problem doesn't cause too many issues for ya - I just reset and keep sailing
