115 Optimax Cooling Issues

I have a 115 optimax that is freshly rebuilt. It will overheat while trying to plane off about half the time light ship, and every time if I have the boat loaded. Once I get on plane it's good to go. I can see on the water pressure gauge that it'll pressure up to about 16-17 psi before the poppet valve will open and cycle and drop it down to about 9 if it will stay open. When it stays open I have no problems. When I can see it cycle is when I know I'm about to get buzzed and knocked into guardian mode. The problem is how long it takes for the poppet to open. If it takes me more than about 5 seconds it isn't going to happen.

I've dredged the internet and see things about restricting the block vent, using a slotted poppet valve, using a lighter spring, etc. It does seem like a common problem that trollers have had. I've replaced the thermostat, poppet valve assembly, and entire water pump assembly all with OEM parts and all with the same results. No luck. The boat is a bit under powered for 3 fat guys and a fishing load but it should still plane off. I'm not one to go questioning/changing the OEM design but it has all brand new shite, plenty of water pressure, and doesn't work. It seems to me that the poppet should open around 10-12 psi but I can easily make 14/15 psi and hold it there and make the buzzer go off very predictably. It makes about 4psi at idle, jumps to 10ish @ about 1200 rpm.

So I guess the question is, any of you optimax owners have some wisdom to shed on this? Any 115 owners had the same problem and found the mystery solution? I'm trying the water pump one more time this week since the last time I put it together I tore a gasket and if that doesn't fix it I'm bringing it in very frustrated. It came back with the alternator wired wrong so I guess it's not too farfetched that something else could be wrong here. I've had the lower unit off about 6 times trying to find things wrong and can't.