engine shock when idling

[Qian Qian <qqian@pppl.gov>]

recently, I notice that I can feel intermittent shock from the engine 
when the car is idling. I kind of remember reading somewhere which
mentioned about engine shock sensor but can't recall the detail.
I first thought this has something to do with gasoline octane which
turned out to be not the case after I switched from special to 
premium(Amoco). My questions are:
(1)is this normal?
(2)could this have anything to do with shock sensor if any exists?
or
(3)is this due to oxygine sensor?('cauz I never changing it since I
got it 3 years ago with 70k on it.)
Regards!

Qian