[Darryl Ramm]

That is a classic sign of a plugin(s) with a denormalization problem.... discussed many times on DUC. You should go looking for the plugin. Just going through the session disabling plugins in batches should help you find the problem plugins, and then you can remove them, reorder them, or stick a dither plugin in front of problem plugins if it is a denormalization problem... but rarer plugin bugs might require removing the plugin from the plugin folder and restarting Pro Tools... so a good quick test is to start by temp, moving all plugins out of the plugin folder, let Pro Tools automatically put back the standard ones and checking if the problem then goes away... if it does not there are still Pro Tools plugins that can suffer from denormalization... but at least then you have narrowed the number to look at.