Ztec s Rev help!!

[Lapo69]

I have just purchased a 2006 Ztec S 68,000 miles and everything ok.. But.. As the car is idle sat in traffic or whatever the car seems to be a little strange, it shakes a little and as I'm looking at the revs as it shakes the needle seems to move ever so slightly, also when I'm driving the car I don't think it's necessarily when I change gear but when I take my foot off the accelerator the Rev needle climbs and then drops normal instead of a steady drop if you get me.. It's had excellent service history so I doubt it will be the spark plugs it's just had a service 2,000 miles a go, any ideas, would cleaning th throttle body help or changing a coil..? Or is it a bigger job?