Re: 1990 PCM 351C missing under load when hot
Your points should actually be set using a dwell meter. Point "gap" is an estimate ONLY of correct dwell.
(see your OEM specs for this)
You'll need to learn whether or not your coil requres resisted voltage, or an internally resisted coil.
A coil that may have an incorrect supplied voltage (too great), may over-heat, causing what you describe.
You have an ignition system that is twenty + years old.
Perhaps it's time for an upgrade to a complete electric triggering ignition system.
Just a thought!
Rick ... aka Ricardo
28' SDN F/B w/ twin Volvo Penta 5.7L DP's