Vauxhall Astra flat spot and idle murmer

i have an astra 2000(X) its only done 37k but its developed a flat spot in first gear at low revs but drives fine above this. Also when stationary idle, the revs dip as if it has a murmer or slight misfire, only happens for a split second but happens very often and very annoying!! The plugs were changed 2 years ago, but cleaned when this started happening, could this be part of the problem? Plus there is a slight blowing/low popping noise from the exhaust (manifold end) when you release the throttle to idle! Please help!!