scoobyady

hi just wondered if this is nomal or a problem . i was driving down the motorway doing about 80 in 6 gear put my foot down to over take an noticed a weird noise then looked at my boost gauge and the warning light was on and it was flickering off the gauge at 1.5 bar. Dropped it into 5th and the same weird noise and the boost gauge was showing the same thing. When i got off the motorway i gave it the beans through all the gears and it was fine just seems to overboost when underload. So is this a problem, any advice would be appreciated??