shaking boost needle

here's my setup 302 tfw tw 185 heads, f-cam,strim, tf intake, S-trim vortech. now my question bebleiove it or not i finnaly justy stepped up to a 75mm tb. with the stock tb my boost guage would climb nice and smooth to 9lbs at redline but since installing the 75 once the guage hits 5 lbs it starts shaking violently up to who knows as it's jumpimg around with about a 4 lb shake. if you know what i mean. I am just wondering if other people have had this happen and/or if it's indictitive of some problem. I just want to be sure everything is a go before i put on the 2.8 pulley and watch the stock bottom end go boom.

thanks guys

thats what I thought at first too but I didn't think it likely as the only thing that cjanged was the tb. But what the hey i'll crank down on the belt and see what happens

I never got to find out if it was the belt slipping or not. I got the 2.8 pulley and within 24 hrs while sitting at a light i heard a loud dink the car started running rough and blowing smoke out one side. I havn't had a chancve to open it up yet but as i still have good oil press and it's not overheating i'm hopeful that its nothing too serious