Frame drops with Dolphin emulator

[MrEnderkiller]

Hello,
I recently downloaded Dolphin emulator 4.0-2213 on my Asus ROG g750js laptop. Here are its specs :
CPU : i7 4700HQ
GPU : gtx 870m (+ intel hd graphics 4600 with Optimus)
RAM : 8 Go

I'm having a problem : whenever I try to emulate a game, the game runs smoothly at 60 fps (with auto framelimit), but I have frame drops which are annoying end even sometimes last 5 seconds. I'd like to precise that my laptop has Optimus technology so the laptop chooses between the integrated graphics and the dedicated graphics depending on the program I'm running, but I manually set it to the dedicated graphics.

I've tried both DirectX and OpenGL, but I'm experiencing the same problem on both engines.

I also have the exact same problem when using the integrated graphics, so the problem probably isn't the graphics card, but I really don't know then what causes this problem.

What do you think ? Which settings should I change ?

Here are screenshots of my current settings : http://postimg.org/gallery/5jciutja/

Thanks in advance for your answers.