lumpy startup on megane 1.5dci?

my megane 1.5dci runs perfect with plenty of power and good economy but on cold mornings it starts first attempt but it is lumpy for may be 1 minute or so, with it starting first time i wouldn`t have thought it would be the heater plugs but may be i`m wrong, has anyone had a similar problem and know`s whats wrong, thanks in advance if you can help with this. regards