it could be a sparkplug failing. to test them use some insulated pliers and pull off a spark plug lead whilst the engine is running. if the sound of the engine gets worse it means that the one you are testing is ok. keep trying on the other 3 until you notice that one makes no difference when you take it off. this is the one most probably failing. remove the plug and earth it on the top of the engine, start the engine and you should see a clean spark at the electrode. to check whether the lead is ok pull the lead off at the distributer and see if it is sparking there, if it isn't, it's your lead. good luck