W140 300SE 92 with M104 engine

I recently developed a hard start problem. It takes 2 times to start rather than the usual 1 time only. Overnight cold engine is fine but if the car have been left after a 30 minutes to 2 hours rest (i.e. warm engine), the start problems comes in.

I have the pressure regulator replaced, spark plugs replaced and fuel filter replaced. What else could be wrong? Advice please.

Many thanks