Sady,

Its true that resistance increases with an increase in temperature. So a hot starter will have an overall higher resistance and draw less current with a reduction of output torque and power. I have seen starters that would start a cold engine but not a hot engine. This type of bad starter, when hot, will draw more current with a decrease in output torque and power.

So if your starter cranks the engine well when hot, the armature and field coils are OK. It may be a resistance on the surface of the contacts points in the relay of the solonoid or the brushes in the starter are worn or have weak springs and are not making good contact. The extra voltage from jumping to another vehicle may be just enough to get enough current thru the higher resistance and get the starter going after which it works OK.

Sorry, I can't explain why it only happens when hot.

P E H