i have a question, since when does an engine produce 30-40 inches of vac.
i have never in my life seen a boost gauge go that low. my autometer boost gauge goes to 30 inches of vac but i have never seen it go farther than 20 in.
if it does bounce like that, you might want to check the wastegate and the wastegate actuator. that would cause an unstable idle, but i still have never seen a boost/vac gauge go that low