ford ka running problems?

hi my y reg ford ka 1.3 endura has a running problem.when its started from cold it revs and picks up fine,when its warmed up if you rev it slowly it picks up ok but if you rev it quickly it runs very lumpy and then picks up and revs ok.when i started it this morning from cold it revved and picked up fine would this be the engine temprature control switch? please help thanks