Let's be more specific on how full your seperator is.

The threaded nipple is the out of the filter/seperator which goes to the IN of your fuel pump. If the fuel level get's just below this, then your pump won't pick up fuel.

So, if seperator seems full, it may actually not be full enough.

Think of this threaded nipple exactly the same as the fuel pickup in your tank.

Maybe the attitude of the boat at cruising speed along with higher fuel demand is when it starts lifting water off the tank bottom. Also when you pull the pickup tube take a good look at the hose to your pump.

Maybe someone put a ping pong ball down your filler ? LOL.

Any old people like me remember the ping pong ball in the tank stories that ran around like wildfire ?
Tons of people I knew at the time said they've done it to others but I never saw any evidence supporting they where telling the truth. LOL.

When I spin the filter off the center part appears to be 1/2 to 3/4 full while the outside ring where the little holes are seems to be all the way to the top. I will have to pull when I get home, but I honestly remember the filter looking like this when I pull it off. I dump everything out and then spin it back on with nothing in it and the motor will fire right up and fuel pressure will climb from 0 to 6 psi in about 5 seconds.

I remember the marbles in ensign pulver, lol

Does the fuel psi gauge show vacuum by any chance ?

I've not read this whole thread but It doesn't sound like the fuel system is breathing, by pulling the filter your breaking the suction, try loosening the the fuel fill cap when it dies and see if it will refire, just a thought

The gauge shows no vacum only the fuel PSI. Next time it drops down to an idle and shows 0 psi i will open the filler and see if the pressure pops back up. I feel the tank is venting ok because when I fill up at the pump and I feel the air rushing out at the vent. Could anything else besides a clogged vent cause vacum?

Collapsed line or antisiphon valve closing. When you crack the filter the line can reopen and same for antiphon valve.

Or, a ping pong ball in your tank closing off your fuel pick up. LOL.
Edit in: debris like a plastic bag / decal/ or etc could do it too. I had a MG here that did this and I finally pulled tank and found hunks of cork in it. I guess MG's used cork for the floats on their fuel level senders.

Fill a straw up with water, put thumb over one end. Trying sucking the water out of the straw.

All lines are 2 years old so I dont think they are collapsed anywhere, however, I will go over them all again tonight. There is no check valve at the tank anymore. It was removed 5 years ago. Unless someone hates me I doubt there is a ping pong ball.