long self could be due to any of these problems ;
weak battery ... not having enough power to crank engine .. but then it would be once or twice
starter motor ... same as above
vapour lock ... one of the possibilities
some electrical component faulty ... failure on heat up is normally behavior of capacitors ... so the problem could be antwhere in ignition, coils, sensors
reduced friction in cylinders : when overhauled engine is not free floating