do you have an anti-virus? maybe it finds something then it starts scanning the process which causes high CPU usage.
Oh wait haha i just realized I have the same problem, its not because of anti-virus or anything, its fgunz fault in the code. Whenever I start moving around and people start shooting, it starts getting so stuttery, and whenever I alt-tab the fps drop a bit as well. The fps stays the same at the counter, but the movement gets really weird and laggy.
Edit2: Feels like cpu usage and fps is good, but when you go to a room full of players, the movement and actions like reload shot and stuff get really stuttery and theres like a small but noticeable delay between the actions, it cannot be my PC. Compared to other servers where movement gets through instantly and you can perform k-style without any delay. It's like the movement is fully server-sided.
