1999 5.8

1999 air nautique with a 5.8 HO EFI w/ 450 original hours. Has run fine until recently.
When cold, engine starts and runs well, but use it, shut it down and it will not restart. A 'service' provider replaced the low pressure lift pump with no change in the result. I proceeded to actually trouble shoot instead of just replacing parts. Checked for water, none. Checked and replaced the filter in the FCC. Checked the tank and pickup for trash or blockage, none. There are no fuel line leaks.
Problem is air in the FCC and the high pressure fuel rail, as I can bleed it off at the schrader valves on the rail and FCC.
Only thing I haven't done yet is actually take pressure readings, and that will get done today!
Ideas?
Anyone with knowledgeable input will be appreciated.