My 94 850 Turbo starts and runs normally up to 35mph then it just has no power. Every 3 second the engine rpm will increase momentarily. Any ideas will be appreciated

I am replacing the turbo on a 1998 S70 engine family B54254T serial 1053902. Taking it apart, there is a gasket between the exhaust manifold and the turbo. There is no gasket between the turbo and the exhaust pipe. Is there supposed to be one? If so does anyone have a part number and a source for it?