I normally avoid posting on these threads, but I think I can help a little here. I've been flying a Venus 2 with a 120 Surpass for a few years. I've been using the 120 for about 10 years. I agree with vbortone that it's overheating and what he says about cowl opening. It will overheat if there is too much air getting in the cowl and not enough getting out. My engine is mounted at 45% with the stock muffler and I have never had a cooling problem. If your head is only sticking out 1/2 inch, that is really only the rocker arms in the wind. You need the cooling on the cylinder. Also the 16-8 is too big. Try a 16-6 APC. As for nitro, I use 20%. I've tried as low as 10% and as high as 50%. It ran fine on the 10%, but doesn't make much power. Anything over 25% and the needle becomes very erratic and you can never finish a flight without landing and adjusting it. I'll stay out of the nitro/heat conversation because I don't know who is right.

Fred