My first

[oldfielder]

Definitely open heat exchanger and see if those pesky chunks made it there. It doesn't take much at all to fry that impeller. Might be a good time to check the risers for blockage. Not fun but the more I find out the more I see that they are another preventive maintenance item.
Great source of info for the mercruisers is boater ed forum. I think it's part of boatfix.com. Very helpful bunch there-kinda like here.
Goodluck,
Doug