96 Johnson 15hp 4 stroke runs very rough after 12 mins idle.

Hi all,
I am at nearly at wits end with this motor, but I WILL beat it hopefully with a bit of assistance. I would be extremely grateful if someone could help me with this.
Starts well, idle good, wot good, idle or run for 12 minutes NOT GOOD. No matter what I do, at about the magic 12 minute mark it run like a mongrel dog.
If I leave it for a while ( 10 mins ,to cool down maybe ? ) it will start well, run for short short time and then back to its crappy self, shaking and spluttering.
Does not change whether I run in tank at home or on the boat in saltwater.
I just get the huff with it and wheel it back into the shed for next time.
I have changed powerpack, stator and coil, thought that I had it sorted when I disconnected and checked oil pressure sensor and hoped but NOOOOOOO! .
Fuel checked, carby checked, all good. Got me stuffed.
Would love to gets the kids on the water this summer down here in Australia.
Cheers,
Stu.

Have you checked the thermostat or pump or any of the cooling lines. Sounds like it gets warm maybe salt in the thermostat or salt buildup in the motor.

Yes I checked thermostat and pump, all good. Water pumps well. I have not thought of salt build up or corrosion on cooling jacket.
I wonder if water could still pump well if the water cooling jacket around motor is blocked.
I have to duck out for a bit but will return an around one and a half hours.
Thanks for the reply

Check valve clearance !