If you don't want to jump right into hardware changes I have had good luck with adding Marvel Mystery Oil to the fuel. This apparently raises the vapor pressure of the fuel.

I believe its the lack of a return that causes the issue. The fuel vaporises in the rail.

ok maybe I'll try the mystery oil first..... thanks to everyone who are putting in
their two cents worth!

My uncle put a cheap primeing bulb,like what an outboard has spliced into his fuel line,it may be a little redneck but open the hatch give two or three pumps and 502 fires right up.

In the VST models, the pressure in the VST becomes higher than the pump can deliver, say 10psi in the VST vs 5-7psi max delivered. The top plug on the VST can be removed to relieve the pressure as a temp fix to get you home. I always leave mine finger tight so I can remove it.

I like the MMOil idea, probably good for the entire system anyhow as long as it is not too much.

I read up on the MMO and it says 4oz. per every 10 gallons.

Would a bypass type regulater and good fuel pump negate the problem?

This has been talked about a lot. If you have the VST tank, take the fuel rail return line off the vst tank and run it to your main gas tank instead. CP makes a nice piece that splices the fuel fill hose and allows you to run the return fuel line to it. Plug the fitting on the VST tank. Mine is a 96 502 mpi, and I had the same problems. I talked to the famous Eddie Young, and he suggested the above fix. Worked like a charm. Not a single issue since.

I have many posts on this and chased for ever. Run a fuel line from the VST back to the tank and cp does have a fitting you can splice in your fuel fill line to the tank. I had a guy weld another bung in it since I have twins. I will tell you this pull your mechanical pumps and check for wear on the arms.. There is supposed to be oil in the reservoir no one ever checks these.. I had one wore.. And the other was leaking. Only had 200 hrs on boat...