We need to fix the octane issue first, then look at the boost. The PCM may be pulling boost to protect the engine from the knock.

Put 91-93 (higher the better) in the car when it'* close to empty. Drive for about 20 miles, then go home and change your spark plugs. Start your diagnosis from that point. Use COPPER plugs. NGK TR55 is a good choice, and gap them at .060" since you have a stock SC pulley.

THEN verify your symptoms. Both seat of the pants as well as gauge info.