I have an 02 four stroke 115 Yamaha. I have replaced the shift position sensor. Spark plugs and had the injectors flushed and new baskets and seals installed. Boat sometimes stumbles then stalls at idle. If I rev it up quick and back off it will do it. If I rev it up and back off slow it will idle fine. Any suggestions? Problem is intermittent and sometimes it will run 5200 rpm sometimes 54-5600. The fuel does have some water in it. I have to drain the seperator frequently. I just bought the boat and I'm unsure of the time it may have sat. Any input would be greatly appreciated.