Oops, was dealing with a similar issue with the Prolific 2303, which was dealt with by using a newer driver, I haven't run into the issue yet with FTDI (which I don't as much).
The funny thing is that we reported this sometime in 2009 (might have been 2010), they "fixed it" and released a new version, but the new version only crashed somewhat less often.
EDIT: just realized that I was thinking of the PL2303, which had a similar issue, not FTDI.