Thx for your quick answer.
Actually I run the game on a HDD 7200rpm. I will receive an M2 SSD tomorrow so I will test with Arma on It.
My goal is not 60. I prefer a constant 30fps than a 60fps which drop to 15 every 30sec.
Since 4 months, the only difference on my computer is the windows 10 April update and I have installed some Adobe software. I also performed a defragmentation.
I just verified the game file integrity, it was fine.
I also tested to launch the 32 version of the game, I didn't notice any difference.
Last but not least, I just activated the huge pages option in the launcher (I'm not sure of the real name of this option, in French it is "Autoriser pages volumineuses") and it seem that the fps are more stable.