My son and I had his 350 Mercruiser engine rebuilt in June. We are having trouble with it. It starts fine, idles, and runs perfect for about 30 minutes. The temp gauge goes about 3/4 of the way to the edge of the green and red zone. (We haven't changed it for a numbered readout.) At high speed, it goes right to the edge of the red. After slowing down it cools off back to the 3/4 position. However, the idle starts to act up and it usually stalls. We cannot get it to restart for at least 4-6 hours when it cools off. I've had 2 shops look at it and talked to the engine rebuilder. (He's coming out to look it over next week.) We've changed the thermostat, points, plugs, wires, condenser, carbs been rebuilt, checked the timing, dwell, and run water thru all the exhaust manifolds and hoses. We rebuilt the lower unit water pump. We've pulled the drive and checked for obstructions in the exhaust. We pulled the water inlet hose after the ouil cooler; it's pumping plenty of water. The port side seems to be running a little cooler than the starboard. The starboard side of the manifold is at 205-210 degrees. While trying to restart, it seems as if it's not turning over fast enough. We have 2 new batteries and a new starter. Does anyone have any ideas?