Your water temp. could be a little high for your engine but I agree with Dan that this sounds more like a coil or some other issue related to ignition. My engines, which are racing engines, run at around 220/230 degrees F which is where they make the most power.
When it stalls and you try to start it, does the engine turn over easy? Also do you notice any roughness in it's running just before it stalls? Will it do this if you just start it and let it set and run for a little bit?
Ron
