[Tom Goldman]

A greatly overlooked area on starters is poor grounding thru the mounts or motor plates and voltage drop on the starter solenoid circuit.
Both problems usually get worse as the engine and car heat up, and show up when you start going rounds.
If you dont have the original ground bond wire at the motormount ,I would definatly put on at least a 4 ga. braided cable between the engine and chassis.
Since its a Stocker ,if you are still using the oem ignition switch ,as many I've seen still do , Eliminate it and since it's a stick use either a high current starter switch with at least 12 ga wire between the solenoid and battery source ,or a low current button with a relay.
I recently saw a Comp dragster with starting issues like yours, the battery term at the starter saw 16 volts ,but the solenoid circuit had such high resistance ,it was only seeing 9 -10 volts at the solenoid , causing it to bounce in and out ,damaging the flywheel.