All dropping nitro will do is add more methanol(the actual fuel) to the mix. Have you replaced the bearings or checked the piston/sleeve fit? It only takes a couple of laps to heat up an engine, so I discovered with a CMB 67. I had very similar issues due to overheating. Ended up ruining a head button and piston and, due to age, a bearing. It would run great, until it got warmed up. Then the button would expand, letting pressure out through the plug threads, followed by bogging down and quitting.