vectra engine light 2?

See if you so called experts can sort this one out.
Vecra 1.6i 2001,car runs o.k.then starts to missfire,engine light on,switch engine off and coast for 5secs,turn back on car runs lovely,then sometimes car missfires no light on,but comes on when you press throttle,foot off throttle light goes out.when pulling away in 1st & 2nd floor the throttle,coughs & splutters a bit,then goes like hell.
When spluttering in 4th or 5th i change down floor it & after a cough its away like hell again.
Please help im desperate.