yes i thought of the masseband -ground cable also,
but since the failire apears only when the Motor is hot ( and so the starter in the near visinity of the exhaust pipe) ist a sure bet that the new starter is the Problem with ist cheap China Switch.


But if the failure shows also when the Motor (exhaust pipes) are cold,
then the most probable cause is what devilkutter mentioned with the cround cable

It is a common Problem with aftermarket Starters that the Switch does not engage when the starter is heated due it´s Position next to the exhaust pipe.