I'll gaurantee it's not the thermostat as this condition only applies when at idle. A thermo either opens or it doesn't. As he can get down to 160 with rpm's then that eliminates that one.

It's also not the circulating water pump, they either work or don't. If they don't then they blow the relief valve and water spews everwhere. They have brass internals so they don't corrode and deteriate.

As i said above I'll bet it's either a blockage due to a large impeller vane that broke off and lodged in the system.

I can't say if a 2000 has a water strainer but if it does it could be blocked or full of debris as well. It would be after the thru-hull but before the raw water pump.

Another possibility if i remember correctly, the transmission cooler. If that's clogged i think it can cause that problem as well. It's a long cylindrical thingy with hoses attached that usually sits down low along side the engine.