Okay here is my problem... After cold starting my 2QM15 everything runs fine for five minutes. Then the dreaded temperature alarm sounds and it is time to shut the motor off. I have taken several troubleshooting steps as discussed here on other threads. First I back flushed the intake side of the water pump inlet. Still overheated. Next I pulled the water pump outlet pipe to verify flow. It has good pressure and just as much water is exiting the rear of the boat as it did when it ran perfectly fine 6 months ago. I filled the cooling system with 3 gallons of strong oxalic acid, waited ten minutes and flushed it with raw sea water.Still overheated. After that failed I removed the thermostat and verified that it is opening and closing at the right temp (It does). I replaced the housing minus the thermostat and pulled both zincs (front and back) dipped them in Oxalic acid for ten minutes and replaced them. Both zincs were about 75% intact. Started the motor up and within five minutes it overheated yet again. Ran a second three gallon flush and rinse as I did above and it still overheats. Not sure what else could be the problem. Any ideas guys?