The source engine is a fickle bitch.
What you're describing sounds like the issue I often have, and is usually the problem causing me to suck (and then get tilted and suck for real).
In source, your sensitivity is tied to your FPS. So if you jump down from 400 to 300, it feels like you just cut your sensitivity a bit, and that feels laggy.
One thing I've found helps is disabling all my other monitors, disabling any black EQ on my monitor (Benq turns it on by default), and letting more natural light into my room.
I have a similar setup, and issue. It also feels more consistent when shadowplay is ON.