25 or less you will start to see stutters. Could be, on descent, more AI aircraft are being rendered, More clouds being drawn and more ground textures are starting to be rendered, AI is the biggest frame rate killer. That is not uncommon on a less than ideal computer specially with an integrated graphics. Turn down your settings is the only alternative to just having a more powerful computer. Have you tried Bujotes tweaks?
CPU: I7 2600K @ 4.7 ghz, GPU and CPU water cooled
GPU: EVGA GTX 780
MEM: Gskill Rippjaw 1600 12800
MB: Asus P8P67 Pro V3.1