the heat exchanger) ?

I've been having overheating issues with my Volvo 2003 (28 HP) and have finally started trying to diagnose them in detail. First, I plan to pick up a basic infrared handheld 'gun' thermometer to verify that my gauge is actually accurate (if the gauge is running higher than the actual engine temp, it might be setting off the electronic switch that makes my alarm work). I think the gauge is OK, but I want to be absolutely sure.

Second, after having read something recently on a diesel engine forum, I realize now that I can also use the infrared thermometer gun to measure the temp of both the 'in' and 'out' freshwater pipes on the heat exchanger. In the thread on said forum about an overheating problem someone had on the same engine I have, a guy wrote [b]"in further testing I found the sea water from the exhaust cool to the touch -- about 80* F. And measuring (with the heat gun thermometer) the temperature on the fresh water input to and output from the heat exchanger, I found the input (freshwater from the engine) to be 200 F and the output from the heat exchanger to be 70 F when the over temp alarm sounded. So, I think there is not enough fresh water flow)." [/b]

I don't see why freshwater exiting the heat exchanger at 70*F would indicate "not enough flow"...any ideas? Should it be cooler? 70* F seems pretty cool already; hard to imagine it being cooler unless in very northern waters, with cold seawater...anyone know why this (above) suggests lack of freshwater flow?

Anyway, I'm thinking that doing this temp check may yield some clues about my overheating...but I don't know what the temp of the cooled freshwater from the heat exchanger normally should be. 70 F, as above, already seems pretty cool to me...

(The engine overheats consistently at 1900 RPMs after it has warmed up.)