Overheating at idle

[The Pirate]

Bought an older boat, which was poorly maintained, and I've been in the process of bringing it up to speed. Boat is an '06 1800 SR3 with 4.3 ltr. When purchased, engine always ran at perfect temp, I watched that like a hawk when first shaking it out. Gauge never moved off of 175f.
I wanted to have impeller changed as I could see that outdrive had never been off boat, not a scuff mark on it.
Had local guy do it to tune of about $400. Since getting it back, and a lot of use this summer, engine temp will spike at idle. Warning signal comes on, temp gets up toward 185f. Usually, just a shift to neutral and add some RPM and temp will drop quickly.
But annoying in that this didn't happen previously. I'm not understanding why this would happen with new impeller. Could this be a thermostat issue? I just don't want to throw money at what is a pretty simple, but annoying problem.
Any thoughts....thanks.

[mmwjr]

I would perform a flow test per the Mercruiser manual where you remove the hose at the thermostat housing that comes from the transom plate and collect the water in a bucket to verify amount of water at a certain rpm for a given time. I suspect the guy that did the job didn’t get a good seal between the pumps output and upper water tube.