Not sure if the missing gasket would cause it as its on the output side of the turbo.
Sounds more like Overboost. The turbo builds up pressure, then it gets too much and dumps it, making the car lurch and the boost drops off then comes back up again.
My classic had this issue. There's a small restrictor pill pipe no 2 and changing this cured the over boost.
Have you changed anything on the car recently, that might increase the boost presuure?
hopefully some one with moe knowledge can give a definative answer.