You've essentially provided all the information to solve your own problem.

I had a volvo AQ131 I/O that exhibited the same behavior. I had an
overheating problem where the engine coolant (closed system) would get up
over 210F. At lower speeds it would hover around 180F and at idle it
would get to about 190F. At idle it would run rough and would have a
tendancy to stall. When I finally fixed the cooling problem the other
problem mysteriously went away, completely!

If you are overheating at lower speeds, change the water impeller. Yes,
long periords of idle will foul the plugs, but that usually just leads to
hard starting and rough idle. What no one pointed out is sometimes
engines running hot will often vaporize the gas in the fuel bowl and cause
the engine to starve for fuel, where it runs rough, stalls, and cannot be
restarted. That is vapor lock.

Since temp alarm went off, you might not have had sufficient water to the
impeller and have now damaged the impeller, or the impeller was unable to
properly circulate the water. Either way, whether it solves your stalling
problem or not, change the impeller!

Bob Dimond