My engine (Nitro Star T-15) has a weird thing about idling. When it comes in from a high-speed run, and idles, it will heat up quickly for a few seconds (around 10 or so) and then start going down. It usually drops about 30 degrees in a couple minutes (yes, and I don't know how, but my engine will idle that long). Wait a while while it is idling next time and see if it goes back down after a minute or so, unless it gets into the Danger Zone (1980's overdriven guitar starts playing in the background ). I believe our two engine are quite similar.

I had a problem with my 2.5 engine with that, but it turned out that the idle was too high. It was my first engine so I had nothing to compare it to, and then I went online and heard how it was supposed to sound at idle. I fixed the temperature at idle problem by richening the LSN a bit and drastically lowering the ISS.

Hope that helps.