Engine derating only revs to 2500rpm

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Intermittent problem sometimes it runs fine but other times it will start fine and run for a few seconds then start to die and coughs and splutters the idle changes to 1000 rpm and wont rev over 2500 rpm no matter weather free rev of under load if you hold it flat it goes to 2500rpm and holds there and coughs and you can hear pops under the bonnet like a quiet backfire