I am having the same problem.
No matter which weapon I use, after 10 to 30 seconds my character stops using the aim and lowers the weapon. New Config did not solve this problem.

On Experimental #0.61.136013 it still doesn't feel as smooth as on stable.
@Hicks - can we expect to get a fix for that before .61 hits stable branch?

I can not confirm, Kayfod.
It is still noticeable in Test #5, at least for me.

It's a little better in Exp Test #4, but still far from .60 stable.
Feels more like 40+ fps now.

I've tried setting the refreshrate on my desktop to 60Hz. It feels a bit better on Exp now.
But still miles away from the feeling on stable.

I had the same problem. Thought it might have been a corrupt config-file so I made everything new and redownloaded the game. Sadly, the problem still existed.