Chuck, have you pulled the risers off to see if they've become restricted? Has it always been a fresh water boat? I've got an '88 Seville mid-cabin with the same engine and had a problem with it running hot at idle the second season I owned it. When I pulled the risers I would venture to guess that they were at least 50% restricted with rust and corrosion in spite of it being a freshwater boat for at least the time I and the previous owner had it (which was 10 years total at that point). I probably should have replaced them but not having any cash at the time I just cleaned them out as best I could, bought some new gaskets, reinstalled them and ran it that way for two more years until I replaced them.
They were really restricting water flow and the extra hot manifolds and risers were adding heat to the block. It's run cool ever since. The only reason I checked them out was on a ride with my in-laws
I started getting a lot of smoke out of the vents due to some cooking exhaust hoses thanks to the insufficient water flow.