I did a bit of research and couldn't find any "fixes".
I would just play around with the settings, turning things like post process off could make a difference.
Make sure you have the latest drivers, and i would always recommend having the latest beta drivers as i never have problems with stability and they sometimes increase performance. (Although, i'm on Nvidia soo.)
It is possible it was the server, (try less than 100 ping servers. Less than 50 is always better imo.)
I would have vsync disabled.
Shadows on high/very high. (High/very high shadows are dealt by GPU not CPU so less load on CPU)
Those are things i can think of from the top of my head. At the end of the day its a mod, optimizations of arma itself are horrible and sometimes they just play up. Many people will say that for a last resort, re install arma and if not that, restore windows. Could be something that is dragging down performance.
Il post back if i find any more info