Frame stutters - what are the causes?

Game runs well on a whole and I have just about every detail on high with no problem apart from this. From my observations it happens alot when you are shooting apart aircraft and they are taking damage. In the background I assume the game is calculating what is getting hit and it has to freeze to catch back up on occasion? That or it's a texture loading problem but usually the stutters are worst at the beginning of a large battle with bullets ect flying about and things getting hit.

What are your observations?

edit : This also happens with small single plane battles too , so it's not a memory issue at fault ( or shouldn't be if coded properly ) A good of fire burst with multiple hits on an aircraft will lead to a stutter atleast .