Think I found the issue. 

The BBCode parser takes exponential time on some posts. I've been suspecting that the 100% CPU issue was caused by my major platform upgrade a month or two ago, and this would make sense since the underlying regular expression engine was probably updated in some possibly minor way that is now susceptible to a near-infinite loop given the same parsing logic we've been using for over a year.

I'll try to narrow it down further so that I can patch the offending logic of the BBCode parser.