While troublshooting ,back to back flushes , will flush one time, next it overflows , I have checked for a blockage in my main line ( none found )

will flush one time, next it overflows , I have checked for a blockage in my main line ( none found ) sure sounds like a partial blockage to me. First flush goes down and begins to build up and the second flush hits the first flush and backs up. The only thing that would cause this is a partial blockage. The solution would be to pull the toilet and snake the drain line. Good luck, Tom