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Your starter may be new, but my money is still on it as the culprit. Are you running headers? If so, how close do they get to the starter? The hotter the starter gets, the more resistance it will build up. It will take a while, 20 min + for the starter to start getting heat saturated.

One possible diagnostic techque would be to get an ice cold rag on the starter the next time this problem arises, and see if it cranks better. OR, the more scientific method, put an ohm meter across it. Simply put the (+) of the ohm meter on the back side of the starter solenoid (starter side) and the (-) to ground. You don't want or have to crank the engine. Compare the readings between hot and cold. To be honest, I don't know what those readings should be, but they should be relatively close to each other.

If it is the culprit, a heat shield may help a bit, but it will also trap the heat in if it's too close to the starter. You may need to have the starter rebuilt by a specialist who can install better windings.

Also, make sure all your connections are totally clean, and that ALL of the battery cables are in good shape. The resistances will add up. Add poor connections to a hot starter, and high compression, and you'll get hard cranking pretty easy.

Good luck
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