Check for a voltage drop along the relevant circuits. Could also be a bad ground or ground circuit.
Start at the battery and the starter motor. The battery post themselves, not the terminals on the cables.
First test the whole circuit to determine if it is the power or ground cricuits, then try sections of the failed circuit, to isolate the exact location of the bad connection.
Cruddy connection somewhere, or dead spots on the starter armature?
At least it's NOT intermittent
regards
mech