I have twin 95 gen 5 502 mag efi. Only 1 of the motors after 20 min of running will stall out. I'll turn key and it will start then quickly stall out for about a half hour then will usually start up and run strong. I swapped the vst pumps with other motor thinking it was a vapor lock. But still does it on same motor. I've also now replaced the mechanical pump, had all the injectors cleaned, replaced the thermostat, sea water pump and water pump. I don't know what else to do! Now every time I go out its getting worse! Any ideas? I even set up another fan in engine compartment to move air

When it stalls does it simply shut off or sputter for a bit then quit?

It just stalls. No sputter

Have a scan done for trouble codes and save yourself headaches and money. JMO. BTDT.

Unfortunately I had it scanned and no codes came up. So I'm just chasing my tail

Did they look at live data like IAC counts, TPS voltage,and etc ?

Swapping parts and pumps won't fix vapor lock if it's on the port motor. Next time it does it take cold water and pour it on the fuel/water seperator. If it starts right up its vapor locking. Quick easy check.

Probably a dumb idea, what about the fuel filter h2o seperator? Doesn't it take a great deal of heat to form a vapor lock?

I had a situation years ago with a bad ignition harness where one motor would just turn off. Scary as hell when running and one motor would just shut off. Replaced the harness and never had an issue again.

I tried that also and didn't help.