What is the vehicle we are talking about?

Got some help from the wife to turn the key for checking the pos cable while double checking voltage it started to spin. It may not be a heat thing but a crimp on a wire terminal at the starter for the solenoid. I will have to bring home my crimpers and fix the end. I will let you know what happens. Thank for all the questions for me to pursue this may have found the problem.

It is a Motor home on a Workhorse

Ok, keep us posted please Bill.

No luck same problem, rechecked the cables neg under .2v positive from bat to starter on the solenoid side when cranking is .49-.51v main cable from stater to battery is .18-.2v not sure which was the B terminal unless I pull the starter. Flip flop starter relay with ac comp relay same thing. It is starting more frequent but I haven't drove 200 miles still need ideas. All the wires to the starter are wrapped in heat tape starter has. Heat shield. I am not sure if it is heat but with the 70 degree temp again today it starts on and off

that .2 v while cranking on the pos side? i might try putting a jumper cable from the body of the starter to the neg terminal

I would still get the starter checked out.

Ok tried the jumper cable to the stater no luck. Added ground from motor to battery, no luck. Added a push button directly to the solinoid, bingo starts up every time. Must be a bad wire or ignition switch. I will know more once it gets hot outside again but that looks like it. Thanks all I will let you know if that doesn't correct it.