I noticed cron had been churning away at 82% CPU for the past 140 hours (!). sc_usage and fs_usage came up empty (with the latter crashing) - so did ktrace. Remembering that “crontab -e -u someuser” hung on “Installing new crontab”, I checked the tabs dir for any temp files. There were many of them. Ultimately, this seemed to help:
$ rm -rf /var/cron/tabs/tmp.* $ launchctl stop com.vix.cron
Back to using just /etc/crontab! :-/