I have a 2006 90 HP 2 stroke Johnson outboard. When idling the overheat indicator alarm and light will come on after about 3 to 5 minutes of idling. If I can take off before the alarms everything seems fine. At high speeds, above 2500 rpms, the warning alarm and light never comes on and the engine never overheats. The water stream seems very strong even at idle. I have put in a new water pump kit, new thermostats, and tried new temp sensors. Nothing has worked. Any suggestions ?