The battery may be intact but nearly at the end of its life with almost no reserve. The battery connections may be corroded (even if they look perfectly ok on the surface). Either of these problems may surface especially in hot weather when resistances of circuits increase. Not a short circuit, a partially open circuit. Monitor the voltage at the battery cables while actually starting. Suspect the above if the voltage falls below about 8.5 to 9 volts with the starter engaged, on a warm day.