I have a gtx 770 2gb, and i have a intel i7 4790k and i constantly get 60 fps and then all of a sudden i drop to like 5-15 frames in games that render like Arma 3, Rust, and DayZ
Sounds like you're maxing out your video memory after some time playing, which causes frame rate decrease. Though when I did play dayz it usually hovered around 1.5 - 1.7GB vram