I have the same problem with Ant Hill (and everything near/above it), the screen freezes every second and its painfull even to walk away from there. Starting a new game did fix it, though. Just not sure if this problem happens with old saves like mine or if it will happen eventually in the game as the days goes by (with more soldiers spawning every day, or something like that). I hope they fix it so I don't have to start it all over again.