I had a similar problem like yours and it turned out to be the valve lash needed adjusting. There was so much back pressure in the engine, that the starter could not turn it over most of the time. This was worst when the engine was warm. To test it, take out the plug (or plugs if you have two) and see if it turns over. If it turns over, that may be your problem. If it does not turn over, that was not the problem.
Good luck