PCM 351 soft seized?

Hello all, new to this forum, thanks for having me. I have an ‘87 Comp TS6M I got from a friend recently. The PCM 351 was recently rebuilt, but has enough hours onit that I’d consider it well broken in. I thought I had a starter issue as it never cranked easily when cold. Having eliminated that by replacing the starter and solenoid and cleaning everything up, I’ve realized it’s actually the motor being quite difficult to turn. I’m willing to consider the thing is soft seized, but that seems a bit unlikely as it runs well when it does and will fire up and run fine if jumped off my truck. Not farmiliar with the drives on these boats, so hoping maybe something else is preventing the motor from turning. Any thoughts would be much appreciated.

Are you talking about a situation when the engine wont spin freely when starting and tends to lobe and stop before moving another half turn? We used to call that a heart attack start.

Are the battery cables original? Been tuned up not too long ago? (cap, points, etc)

Have the battery load tested to rule out a weak battery and check all the cables. After that, I would check the timing; Perhaps the distributor was reinstalled one tooth off. That will make for hard starting.