Often times there is deposits in the poppet nozzles. This can sometimes be corrected by cleaning the injectors. The trick is finding a chemical that will break down the deposits. The stuff that is added to the tank is usually sub-par. Most often it takes something like a top engine cleaner, diluted with gasoline, and fed with a pressurized can. This is one possibility, usually occurs more cold, and diminishes as it warms, but not always. If it is missing under load, it may be ignition related, plug/wire etc.