Without knowing your computer specs it's hard to come to a direct diagnosis. However, what you appear to be describing is jolting, otherwise known as a sudden FPS drop. This is caused because of the stress caused on the CPU.
Because Arma, is porly optimised when it comes to GPU usage, the CPU and RAM get hammered pretty hard (more so the CPU). What you should look into getting if you haven't already is a SSD (Solid State Drive) you can pick them up for around £20-30 (depending on the size you want) An SSD runs a lot faster than a standard Harddrive.
Seeing as you're only experiencing the issue throughout gun fights (and perhaps when you're driving) it sounds like it's down to entity rendering. A SSD should eliminate this issue or minimise it at worst.
The Intel Core i5 series is the best for Arma gaming. The i7's are good too, but they mostly focus on Hyperthreading which isn't made in much use with Arma (but they will help if you have a tendency to Stream + Record)
Also, as a side note. 10GB+ of RAM will also help you out (I have 16GB 1800mHz)
You would benefit from having a CPU, GPU and RAM usage monitor running to see what maybe bottlenecking.
Hope I could help, if you need anything else. Send me a PM/reply.
Kind Regards,