This is my first turbo car so please bear with me...

Background; 2.2 1989 GTC Lebaron, M/T, Garrett stock turbo, new cylinder head, recently remanufactured turbo, the only issues I am aware of are minor oil leak around the PCV valve + TPS fault code 24 (TPS is new and also SMEC/ECU, but thats a separate mystery I guess). Low oil consumption. Fuel consumption normal. I have changed ignition timing to just slightly earlier than when car was checked at shop - this is because it didnt run good (shop didnt have exact equipment so I am not sure they were right anyway).

Anyway, this question is about the turbo overload. Acceleration is smooth and nice from vac to boost, specially with the boost gauge at 2-10, but when increasing boost the overload protection kicks in at 12-15. Which in it self is normal (and afterwards, I get a new fault code for turbo boost excess=OK).

What I am wondering is about the way the protection kicks in - brutal. The whole car shakes, as if I had instantly stepped/released brake pedal. (It was there before turbo reman too, and also with other ignition timing).

Is this normal? If not, theories on why? Something I can do to get a smoother protector?

Thanks from Sweden