The problem is heat related as you have determined. Heat goes up, so does resistance and subsequent amps required.
1) Check all electrical connections to make sure they are clean and tight. Want zero resistance due to connections.
2) Run a separate ground from starter to battery. Just as important to get power return negative side circuit to be direct path as the positive circuit side.
3) Run lower gage (bigger wire) positive cable. A lot of cables are 4 ga, you can run 2 ga or even get a custom cable made using welding cable in 0 ga. Bigger is better!
4) Check timing, if too advanced the engine struggles with the combustion load against the starter.
5) If all else fails, you can put an ignition cutout switch. You turn ignition off, then crank engine to get it rotating over, then flip the ignition switch and it will fire right off. Only needed when hot or cranking too slow.
2005 Kenworth Showhauler truck conversion.
. 44 ft, Cummins ISX15 450 hp, 10 speed, twin screw
I used to have a handle on life, but it broke