The heat could very well be effecting the engine. Since you're not in a real marginal power situation, going a few clicks richer may not hurt in that heat (we've had the same heat here in Kansas also). But since you had good throttle response at 1/2 and 1/4 throttle, and not a full throttle, I'd look at the idle mixture being just a bit rich, or possible a fuel draw issue. You may want to check your fuel system to make sure you have don't have any kinks or holes in any lines. I recently had a problem with a plane that started running lean in flight and quit when I throttled it down. Found the pressure line had torn at the nipple on the muffler after I put it on.

I'm also assuming your engine is fully broken in. If not, you could need some more flight time on it before you'll be able to get the idle mixture dialed in. Been there, done that several times.

Hogflyer