my guess is the fuseable links are corroded. these are at the starter wiring connections. they are part of the wiring and look like stubby pencil shape an inch long. there may be another link running across the intake manifold also hidden in the loom covering.
I have found when cold no problem, but after the vehicle heats up this then occurs and these links are the problem.
my theory is the links heat up with the voltage drop. the longer the operation the greater the heat . as the temp increases the resistance increases.
make sure all the fasteners at the starter are clean and secure including the one with no wires running to it.[solenoid to armature]