2005 Challenger 180 Overheating??

[skeenen]

I recently bought a 2005 180 Challenger with 100 hrs and a 215hp Rotax engine. First experience with the boat resulted in sucking up some rocks, which led to having the impeller and wear ring rebuilt. Got back out on the water and after running for about 10 mins the warning signal for overheating sounded (continuous beep) and the low oil light came on. Engine was running but roughly. After shutting down for 2 - 5 mins it was okay. Oil level is fine, and coolant level is good. We next put in new plugs and I topped up the tank with 5 gals of premium gas. So back out on the water again. Got up and running, everything fine cruising about 20 mph for 20 mins, when throttling down to stop the overheating indicator (continuous beep) sounded, engine ran rough (did not see the low oil this time). Shut down for 2 mins and started back up. Powers up fine, ran for 5 mins, throttled down and same thing happened. Can run okay at slow idle, it just seems to happen now when powering down from cruising. Any ideas on what this might be?