How about capturing some regular snapshots with vmstat? Maybe
vmstat -d 60
piped to a file for a few days (or at least a good bit of time before, during and after the event in question). (These commands may require different syntax if you're on FreeBSD, which I can't test with -- we're basically looking for VM stuff and IO/disk stuff... also see iostat) Maybe also throw in a few vmstat -s's for good measure. This would at least provide a little bit more detail around swap in/out and IO.