Same issue. All ultra settings @ 1080p, no MODS.
My fps will drop from avg 40 on multiplayer to 5 fps after about 1 or 2 hours. I've had it happen a number of times now and have no issue with any other game. Also no one else is having this issue on the server, at least not at the same time as me. I have heard of others complaining of the same issue in the last 24 hours though. This leads me to think it's an issue with Arma and the server I'm on.
Specs- 4790k no oc, 980ti factory oc, 16gb ram ddr3, arma installed on samdung 850 ssd. Arma log- https://drive.google.com/open?id=0B2gJejXSrLAfeHhQNEJXUTVxQ3c
15 second video of problem-