Question for you, what is the average and max CPU utilization during gameplay? I know I had (and still do in some cases) have this issue despite graphics settings. Granted, I understand I'm running older hardware, (Q6600 OCed to 2.8) but I've had this issue as well and I'm positive it is caused by FSB/CPU bottleneck.
All 4 cores for me are over 88% utilized despite .ini and graphics settings.
EDIT: To give you a bit more information, I mainly run into this problem when the draw distance is higher (large maps) and there are more players (48-64 player servers). In these cases I can hit 96-100% utilization momentarily which causes a bad stutter and framerate loss. Also, like you experience, when I'm scoped this is less of an issue.
For me it seems that only solution is to have a CPU (6/8 core) with a lot of bandwidth coupled with decent speed (3Ghz >), this game's physics and scale just eats processors for lunch!
Intel Q6600 OCed to 2.8 Ghz
8 GB Corsair Vengeance (1333 Mhz)
ATI 6870 GPU
If it is asymptomatic in regards to your CPU, that puts a bit of a kink in my ideas. 60-80% should be acceptable in regards to stutter.
I never really had the problem in campaign, but I used to have more of a continual studder until reduced my draw distance in the settings file, removed blur, dropped my resolution from 1920x1080 to 1600x1200, and updated my Catalyst drivers - that actually did help reduce some of the stutter for me.
Otherwise I'm not too sure what else you can do based on my experience, sorry mate.