I have not had a starter 'heat soak' problem on 123s like I experienced on Ford pickups.

Since you just replaced it look to some of your work. Carfefully re-examine the high current lugs which supply power to the starter.

Explicity examine the ground return strap.

If it is cranking slow try a jumper bypass for the above high current runs using a jumper cable.

Finally never forget the new starter could be faulty.....

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Dionysius