surge of revs on renault laguna 1.9dci 2003?

Hi,my laguna 1.9 dci estate 2003 has developed a surge of revs.When you accelerate quickly up through the gears the revs seem to surge really high before dropping really low and then eventually evening itself out.You can actually watch the rev counter going up and down then coming back up!!It has recently had a new oil intercooler from renault costing almost £800!!Its only when you accelerate hard through the gears and doesnt happen if you take it easy,any suggestions or advice greatly appreciated as i know very little about cars,thanks.