I've tried the following troubleshooting steps so far, one at a time, to resolve the issue:
- Ran Diskwarrior diagnostics while booted from that DVD (repaired some problems on internal drives, but ran out of memory when trying to diagnose 2TB external backup drive; afraid to try it on 4TB Drobo as it's specially formatted)
- Swapped out RAM
- Disconnected Drobo
- Turned off Time Machine and disconnected 2TB external drive used for backup
- Purged system crud with Yasu
- Closed most tabs in Firefox
What to try next? Total reformat and system reinstall? Ugh...