Could be bad starter, how old it is?
As starter motor heats up it draws a few more amps, that is normal, but if it requires too many amps that would indicate bad motor and it will get worse as time goes by.
Another issue could be bad battery cable(s), they suffer the same fate over time, they pass less amps as they warm up, the alternator feeds power back to battery via the short red battery cable that goes to Starter Relay on fender, so that is the most likely one to go bad, but it could be the longer one from Relay to starter motor as well.
And could just be those connections, remove and clean.
I wouldn't think Starter relay because that would tend to have the same symptom cold or hot, but..............they do go bad because of the high amps they have to pass each time you start engine.