Irregular engine vibration

[polo-sib]

My 1.0i engine when running seems to do this

ts---ts---ts---vrrrrrrr
ts---ts---ts---vrrrrrrr
ts---ts---ts---vrrrrrrr

At first it was a lot worse till an oil change but you can feel the car vibrating irregularly (and alot more at the vrrrrrr part). Can anyone think or know of a possible problem?

Thanks in advance