2001 Vauxhall Corsa Engine lacks power

Engine Performance problem
2001 Vauxhall Corsa Two Wheel Drive Manual 44000 miles

This is a 3 cyl, Envoy 1.2l

When cold, the engine hunts quite badly, and if driving, accelerates/decelerates like kangaroo petrol. There is a light smell of petrol vapour in the car as you start it. Thereafter, it seems ok. At low speed, the eingine hunts and when the throttle is pressed lacks power. Engine seems to lack power all the way up to around 3000rev.

I have replaced the coil block that sits on top of the engine, and I have checked though not replaced the spark plugs. They are clean, with just light brown deposits.