Roger.... well, 5 psi cold is gonna be 2-3 psi hot. But it could have just been the main already being damaged. Have you taken the oil pump apart and measured internal clearances and inspected? When you do, look carefully at the bypass plunger and see if it moves freely. (It will be quite hard to push against the relief spring when fully assembled.)
FWIW.... I would expect to see rod bearing problems the same or worse than the mains if it was consistently just oil pressure failure. May I ask what was used to check the main bearing clearances originally?