2013 - 5.7 liter dies after 20 minutes - getting air in the fuel line

Hello I have a mystery with a essentially new 2013 5.7 Marine power with fresh water cooling on 2004 Chris Craft Launch. Engine runs for 20 minutes then dies. If you keep the engine at 2500rpm it will run great for hours. After stalling and sputtering to a stop if you let it sit it runs again for another 20 minutes.
Thus far we have tried these remedies.
1. New fuel lines
2. New Fuel filter
3. Pulled fuel tank line and sealed all tank valves
4. Told we might be vapor locking from the fuel heating up so we added a raw water fuel chiller
5. Thought low pressure fuel pump might be the culprit - replaced it.
6. Replaced high pressure fuel pump
7. Replaced fuel regulator
8. Hooked external tank with clear lines to the engine still got air in line from engine side.
9.Getting 45 PSI when engine is started then slowly air gets in and pressure drops to 20PSI

Any ideas?

Sounds as though you are having a similar snag to mine on my 2008 5.7. I have done all the above things mentioned above. George at Marine Power told me to replace the fuel regulator, I did and initially the fuel pressure at the fuel rail measured 65 psi, took it to the lake and mysteriously the pressure dropped to 52 and boat hesitated ( problem that began first of the season and got worse.) replaced high press pump with new one and problem with fuel press got worse??. Tomorrow I am going to remove the smaller braided line from the return on the fuel reg, plug the hole in the canister and see if there is fuel returning to the pump. There shouldn't be if the press is only 48-50 right now. If there is then the new reg is duff.
I am at wits end as the problem only began this year after winter storage (enclosed garage, full fuel, stabilizer). The engine lost power after the first hour on the water and then wouldn't respond to throttle and chugged and died, it would sporadically do this from then on in and I have chased this thing methodically. I have done everything I can think of. Maybe the new fuel pump is a bad one? I will let you know if something is found