When bitcoin is exhibiting this slow-down behavior, it appears to be requesting something from the "DirectoryService" process (user name, reading a plist?). I'm not positive asto how to read these stack traces, but the issue seems to be within this block of code (perhaps it is running a lot more often then anticipated, contention over dbenv, or the db system is making ineffecient use of some DirectoryService).
Is your Library/Application Data/ directory on a network-mounted volume?
Running bitcoin 0.3.13 on my OSX 10.6.4 Mac I'm not seeing any issues (it uses almost zero CPU when it is not generating, and 100%, but nice'ed, when it is). I don't see any DirectoryService CPU usage.
From what you've said, it looks like disk writes are chewing up lots of CPU time for some odd reason.