I've been fighting with intermittent cooling issues in my 2Q15M for probably six months. I need advice on next steps.

Here's the situation, excuse the length.

It all started about 6 months ago - no water discharge at the dock, temperature alarm. Opened up the water pump and the impeller was cracked at the brass bearing - all parts recovered. Back together, pumped fine, went out to race, ran fine. About 5 minutes after starting to return to marina, no flow.

I've gone thru two paid "mechanics" who have basically done nothing but get her running/pumping again and each time AFTER RACING, no water discharge, engine temp alarm.

So, after taking things into our own hands, the crew has confirmed the impeller is okay and via testing confirmed that we have positive flow from the kingston cock to the thermostat assembly, at each segment. In the process of removing and testing flow we felt like we had a lot of old/bad hoses so everything along the bypass route, including the thermostat has been replaced. Thank goodness for our local oil and gas hose supplier as most of the needed hoses are no longer manufactured by Yanmar.

After all replacements she primed immediately and over two days we ran her two 30-minutes sessions at the dock, in gear at 1800 rpm with good flow. The engine, however never got to the operating temp per manual that would open the thermostat (air temps in the high 40s, water temp 57 degrees) so we don't know anything about flow thru the engine.

So, this Saturday we motored out for our season ending regatta, good flow all the way out (only 15 minutes, max). Had a great day of racing and within 5 minutes of starting back up to return to marina no water flow, engine temp alarm. On me but I did not check for flow immediately on starting, there were main flaking hijinks going on.

So, what next? We'll of course recheck the impeller but ... Restriction in mixing elbow? Issue with exhaust manifold? What??

Help, please.

Frustrated in TX.