Suspect overheating: Mariner 9.9

[WallyMaster]

My 1981 Mariner 9.9 runs excellent at idle (can troll all day long), but when accelerated to full throttle, it runs full speed for about 15-30 seconds, then progressively bogs down to about 70% power and remains there. I have cleaned the carb, new fuel pump and filter, new fuel lines and connectors, verifired gas vent is open. Solid water stream out peehole; however, there is only a slight mist of water coming out of the exhaust relief hole on the leg. There is slightly more water coming out this hole at idle, but at full speed, it is almost nothing (only slight mist). This makes me suspect an overheating issue... at full speed shouldn't there be lots of water dropping out of that exhaust relief hole (as there seems to be on my other outboards). Is a solid pee stream out the telltale necessarily indicative that the engine is being properly cooled, especially when there is only a slight mist out the exhaust hole? Thanks