It may be the device that brings tension on the chain that drives the camshafts. (not the chain that you always see in the open air, this chain is INSIDE the motor) It does not always click automatically to the next position. If you remove the bolt just under the entry of the carburetors on the backside of the motorblock you can push gently with the tip of a screwdriver in order to make the tension device. When it says "click" the problem may have disappeared. It is better/safer to ask the garage as the cause may also be wear of the chain. (life over 70.000 km) It may also be valves. Have to be tuned each 24.000 km.