I am very familiar with internal combustion engines with this kind of problem in general but have no experience on Lycoming. As others have said a lazy starter can cause this but odd it's ok when cold but not hot. Does anyone know if the Lycoming has any valve operated decompression mechanism to lower compression for starting as seen on other piston engines. If so that could be the problem. All it takes with this system is wrong valve clearances and you get this result.