You got an app that is a cpu hog maybe, it could be a number of things. Reboot your phone and then clear out all your tasks using an app like Fast Reboot, then run a quadrant. Also an app like system monitor can show you what apps are eating up cpu and battery. Hope this helps.
I just noticed I got the same set up as you as far as kernel and ROM, its got to be an app in the background cause I get no lag whatsoever and im only over clocked at 1.5 with the screen on.
Edit again: maybe your over clocked too much, I read that pushing it too high can create lag also and be counter productive. With me 1.5/on demand seems to be the "sweet spot" for battery/performance.