Paul,
You can also rule out the head and the balancer. There is no physical condition in the head to cause what you originally described. A stock balancer with some wear might slip occasionally under load more so if all condition were right but usually without the same results as what you are posting. The ATI balancer by its design would make it rather unlikely to slip if installed correctly. The hardest thing determining a solution to your question is the inconsistency you report that it is a gradual increase to your normal maximum boost which eliminates many of the the good suggestion to its cause.
