This situation may have arisen if you have recently disabled Intel Pentium4's Hyperthreading. Pro Tools does not work correctly with Hyperthreading enabled, and the CPU usage limit (in Setups - Playback Engine) can be set to higher than 85%. These settings really are not valid settings, and when you disable Hyperthreading, these settings go away, but sort of 'get stuck'. All you need to do is simply choose 75% CPU usage limit and then choose 85% again, hit OK, and the problem should be resolved.