Cars Q&A

I have a Chevrolet Optra (1.6)petrol,2004 model. It has run 60,000 Km of which the last 30 were on LPG. The car has now developed a problem that in the morning (whenever the engine is cold), it is unable to maintain idling RPM, the engine stutters and lightly misfires and shuts off. Once it warms up it performs perfectly. I have taken the car to the chevrolet garage but they are unable to identify the problem since the car is warmed up by the time it reaches there. What is the cause and remedy for this problem?