How much have you driven since the exchange? And - you just changed the rings - or the whole set (+ cut)? Anyway - now you are running the engine, so you should lubricate more (approx. 1:20), do not keep the engine at a stable speed, do not drive it at full power and do not do "long" shots - so last at least 500 km, then you can slowly load the engine more (more approx. 1000 km) - if you do not comply, you can seize the engine. ... "It 's possible they're working hard and it's overheating" ... Yeah. It is also possible that the carbon is only being cleaned. And it's possible that it's sticking to you. I guess - being you, the engine has disassembled again and checked everything again, including the anti-knock gap, the tightness of the cylinder head ... And don't forget to clean the exhaust. ;-)