I have 2 suggestions you can try
First is to try using the FPSlimter
(DL LINK) unpack the linked file and run the .jar file from it to load the gui link the fsx.exe to a bat file and limit the framerate (in the gui) to 30 then in FSX set your framerate to unlimited this moves the framelimiting to and external app which does a much better job & will stop FSX grabbing hold of every resource in your PC it totally removes the bounce between high/low FPS
2nd suggestion is to try
AddonConvertX a payware ulility that converts those hack ass pre sp2 sdk addons into compatible dx10 addons either on the fly of via preprocessing they have a demo for download which is a 10min time limited affair worked for me while i used dx10
hope that helps
Laters
Gizmo