7.4 Running hot need help

[Casey]

I have a 1997 SCR 2700 powered by a 7.4 carbureted. I've owned this boat for about 1 year. Last time out on my way back in in noticed my engine temp was near 200 deg f. Normally it runs right around 150. I throttled the engine back to and idle and the temp held at about 200 deg. I then shut off the engine and restarted in less than a minute. Temp at and idle dropped right back off to normal. Started underway again and the temp climbed. Back to and idle no drop in temp. Shut engine down and restarted and the temp dropped to about normal again. Eased my way back and trailered the boat home. I replaced the impeller before my first trip along with other maintenance before going out this season. But I had kicked up a lot of sand a couple different times on this trip. I pulled the impeller and changed it even though it look fine I also changed the thermostat not knowing when it was changed last. When I had the thermostat out inside the intake manifold under the thermostat I pulled a chunk of rust out about the size of a silver dollar and curved in shape. Reinstalled the thermostat and ran the engine on a set of muffs. Idled for about 10 minutes and Temp was good. Brought engine up to about 1300 rpm and then temp climbed to about 195 and I idle back and shut down. Checking the exhaust manifolds the port manifold I could put my hand on no problem. The starboard side did not take long before I removed my hand. Could it be I have a bad exhaust manifold that is causing my overheating issue. And if so can someone explain to me why. I am having a had time rationalizing it. If it is not being caused by a bad exhaust manifold any suggestion would be appreciated.