This running-wild behaviour is a well known problem with the somewhat broken OSX OpenCL implementation and the phatk-style miner kernels. No proper solution yet. However, you can use an earlier version of Diablo miner with a different kernel. If you check out the sources, do a "git log" and look for the time the switch to the phatk occurred (iirc Jun 24), and use that one. You can also find an older version packaged in a Mac-friendly app by someone else here on the forums: http://bitcointalk.org/index.php?topic=8994.0

This really should be an FAQ entry somewhere...