2003 F115TXRA Overhating

My 2003 F115 was running like a champ last weekend when I took off from the ramp, got about a mile out running at WOT and the alram started going off. Immediately shut the motor down and let it set for a few minutes and tried to start it back up. It would not start just cranked over and over. Let it set for about 30 mins. then tried again and it started and ran fine. Water was coming out of the tell tale so I hoped evrything was fine. Shortly there after the alarm went off again and no water was coming out of the tell tail. Long story short dropped the lower unit and replaced impeller. Ran it again today in tank and no water from tell tail. Hooked up hose to the flush hose on motor and got water form the pee hole. Cranked motor and got good pressure. Shut the water off with motor still in tank and all was well til I reved motor, lost the water from the pee hole and shut motor off. Sorry this was so long but any help would be greatly appreciated. Thanks, Derek