I don't know why this happens but when in some addon planes my fps is ok when I just come in the plane but when I switch to external view and come back to the VC the fps goes down and the sim gets jerky. Anyone ever experience this too?

FSX has to reload all the textures when switching views, so it may take a while for your computer to accomplish this task and resume normal function.

The more detailed an aircraft, the longer it takes to render all those textures and when you switch views, FSX must redraw everything.