What kind of fuel are you running? What is your base timing adjusted to? It sounds like your knockbox is kicking in and retarding timing.
Try retarding the timing a few degrees, and if you arent running premium fuel make sure you do so.
Also, are you going by the stock gauge? It isnt very accurate at then end of the scale. SunPro makes a boost gauge available for around $25, and most autoparts stores carry them now.