I have received the same problem except I am on a 64-Bit up to date windows 10 with the newest drivers running for my GTX 970, Arma 3 is 32 bit and thus cannot handle more than 4 gigs of System Primary Memory (Ram) and has "unforeseen consequences" if you have more than 4 gigs of VRam under your GPU's belt but can handle a maximum of 8 gigs of VRam, a good way to counter this is to trick Arma 3 into thinking you are running those max specs in the launcher under the Parameters \ All Parameters tab \ Advanced sub menu. also while you are there try to see if that hyper-threading option will increase your performance.
help with finding setting in the launcher
https://1drv.ms/i/s!AmH4OS3FoUUkh02p4h7-eI6X_oSX