1999 Renault Clio engine running lumpy

hi i have a renault clio 1.4rt its running a bit lumpy like when u put your foot on the throttle the revs dont pick up very well i have check for spark all the 4 plugs have spark but the AA man had a look said it was running a bit lean and he said there a blow near the front of the exhaust just before the lamda senser would that cause the car to run lumpy and do u think it could be the ecu giving problems? just had another look found out it was the timming belt slip causing the engine to run lumpy