Lucas,

When was the last time the power cables to the starter were replaced? Sometimes the cables start to have strands break inside the insulation due to flexing and vibration. This reduces the effective cross-section and increases the resistance.

Have you checked the cable fittings where they attach to the solenoid and the starter? If they are loose it can lead to arcing and overheating, again increasing resistance. If they are corroded they may not make good contact with the bolts where they mount to the starter or relay.

That's where I would start looking.

Mike Bamberg