Merc 150EFI intermitten overhead alarm HELP!!!!

I own a 2002 Merc 150 EFI and run it on a Skeeter SX190, and for the last three seasons (including 2008) I have had an intermitten problem with the overheat alarm sounding. In the beginning it happened only while getting the boat on plane (hole shot) and would go into Guardian mode. After sinking $2,000.00 into the motor including repalcing the waterpump (impeller), thermostats, sensors, and most recently the Alternator. The most recent work the mechanic diagnosed a power surge from altenator which burnt out the key switch and voltage regualtor (built into altenator), and replaced both. I contacted Mercury and they think it could be a harness issue! Does any experienced Merc techs have any ideas or advice before I sink any more money into this wild goose chase????? Any help is appreciated!!!!!