Stutters to me are micro pauses every say 3 to 10 seconds lasting about half a second, and seem more GPU.
They 95% went away when i upgraded.
The bigger drops in FPS can happen any time, online or off, and are just that...a big drop in FPS and a pause of a second or two for what seems no reason.
It happens in busy offline missions or just flying along, or online just sitting on the runway.
they seem more CPU than GPU to me.
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Furbs, Tree and Falstaff...The COD killers... 
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