2000 Vauxhall Vectra Engine shakes when on idle

when the car is on idle one minute it runs fine but the next minute the rev counter wavers and engine shakes, its as though its caughing. When pulling away it doesnt feel like there is a lot of power and starts kangarooing (it aint my clutch control). i have been told it is the timing chain but surely the problem would be constant which it is not. It is bad enough for me to be getting rely fed up with it.