Engine dies as it warms up - Why?

[JWehrheim]

Has anyone had this issue? Engine starts fine and runs, but the engine will begin to stumble once the motor gets up to around 120 degrees. Motor idles around 600 rpm, volts drop from 13.3 to 12.9. Does the engine die due to temp or due to volts. I would think it's not due to volts, but due to temp.
For various reasons I have replaced a lot of parts.
Engine is a 5.7 Marine Power in a 2008 RZ2
Battery, Rotor, Cap, Plugs, High Pressure Fuel Pump, Pressure Release Valve, Injector Seals, Throttle Body Intake Gasket.

[Poorbunny2]

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