1998 15 hp Evinrude 4 stroke will not run more than 15 minutes

OK I bought this boat in late March. Wanted to make sure the water pump was working correctly. Put it in a tub of water and ran it and could see I needed a new impeller so I changed that. when the ice opened up on lake in early April I took it out for a test run and it ran good for about 15 minutes then spit and sputter then shut down. priming ball was hard and not start again unless I left it set for awhile. So I brought it back and pulled the carb off and cleaned it out then cleaned fuel filter screen Didn't see anything bad in there so I figured I would try it again. This time with new plugs and a new fuel line and priming ball. Same thing happened. It will run wide open or just idle for trolling but will shut down and I have to wait for awhile for it to run. Now the water is pumping fine and doesn't seem to be getting hot but it seems to be shutting down after it gets warmed up. The model number is E15RL4EEB . Not sure what I should try next.

Check your spark when it quits. Should jump a 3/8" open air gap on a spark tester.

I will try that when I get out later this week. Thanks

Ok I ran it in a barrel until it shut down and tested spark. It seemed like the top cylinder wire was hesitant to spark as before I ran it.

Would the spark jump a 3/8' open air gap on both cylinders when it quit ??

Yes it would and thanks for helping me.

When I was running the motor in the barrel for the spark test there a couple weeks ago I had the cover off and the motor ran longer than it normally would run. About 40 minutes so I put the cover on and it shut down in five minutes. I took the boat out on the water a couple days ago and ran it with the cover off and it ran for hours with no problem. What would be affected by heat and cause it to shut down?

Squirt some oil into the engine when it is running .----Look for an exhaust leak.

Has anyone else experienced this problem with the 9.9 and if so did you find a fix ?