I was told that 20% Nitro will run cooler than the 15% mix. I was thinking just the opposite. I was wanting to try the 15% fuel mix because I think my motor is getting too hot, causing it to stop running.

There's 2 ways to cool an engine with the fuel mix... more oil or less nitro.

The easier way to cool it (assuming it has adequate airflow) is to open the needle valve a bit. You should be tuning for maximum RPM then opening the main needle appx 1/8 to 1/4 turn.

Not a normal problem for a trainer.... If the engine is heating due to being cowled and not having adequate airflow. Changing fuel or tuning the engine won't help for more than 10 extra seconds. You need to ensure you have at least 2.5 times the exit air opening size as entry opening. (The preferred ratio is usually 3 times as much exit as entry.) You also need to be sure the airflow route will have the air going over the engine head and cylinder. The head and cylinder need 9 times the cooling airflow that the crankcase needs. (can't have the crankcase get 0 cooling air...)

I have an MDS .40 on an Extra Easy II. I don't know if it is a heat problem or maybe it is a bad motor. (All the older veterans at the airfield have very little good to say about MDS.) I am running 15% Nitro fuel, the engine is totally exposed to the open air. I have flown this motor for almost a year now. I have maybe 20 to 30 flights on it. I am having a heck of a time getting the motor dialed in. Even with the help of a few Veterans with 30 years of experience we have struggled to get this thing running right.
It constantly dies in the air, and seems to load up on taxi out to take off. We have tired too rich and lean settings, nothing seems to work yet. It runs great setting on the bench, and when I point the nose up it continues to run great. When in the air, it dies at 1/4 throttle. I have more dead stick landings than I do with the motor running. I am thinking heat, is the issue. This is why I want to try a fuel mix of 10%, might it help?