What is your head shimmed to? What nitro content are you running? That engine comes with a standard head button. So someone has changed it, and could have removed the shims. You should have 3 on there I have to check the thicknesses. It sounds like you have too much compression and or a stretched rod. First place, I would check is the head shims, take them off, clean them and mic them so you know how much clearance you have / or don't have
I'm shimmed for 30 but run Byrons 25 - good stuff and has plenty of power.