Miller Legend welder. Onan engine starts and runs a few minutes then dies. Why won't it restart for 5 minutes?

Have you stripped and cleaned out carburettor?
sounds like a blocked idle jet?

To clean the slip rings on your 1995 Miller Legend gas welder, you will need to follow these steps:

1. Locate the slip rings on the Onan engine. They are typically located near the rotor of the alternator, on the end opposite the drive shaft. They will look like small, cylindrical pieces of metal.
2. Turn off the welder and disconnect the power source.
3. Use a wire brush to clean the slip rings by gently brushing away any dirt, debris, or corrosion. Be sure to brush in the direction of the rings' rotation.
4. Use a clean rag or compressed air to remove any remaining debris.
5. Reinstall the brushes and check that they are making good contact with the slip rings.
6. Turn on the welder and check that the machine is running smoothly.

It's important to use caution when cleaning the slip rings, as they are a critical component of the electrical system and can be easily damaged. It's important to check the manual of the welder for troubleshooting steps and also check the warranty of the welder and see if it's still covered by the manufacturer. It's best to take your welder to a qualified Miller Electric welder repair service center, they will be able to diagnose and fix the issue if needed.

The Miller 212 has a 60% duty cycle. So for every ten minutes of welding the machine is designed to weld for 6 min and cool down for 4 min.

You may be having an intermittent failure with the fuel pressure regulator - checking the fuel pressure when the truck won't start may pinpoint this problem. If the pressure looks OK, this same problem can be caused by the ignition system as well (ignition coil or control module).

possiable coil or fuel filter and or fuel pump

change the fuel filter and check the fuel pressure

CHANGE THE SPARK PLUG.

fan or relay problem welder is protecting itself from burning up when it cools off will run fine check relay or fan

If the fuel filter (strainer) is OK, carb gives enough fuel for the load you apply, air filter is dry and clean, your problem may be the low oil level protection.
Check if generator is at level position.
After the oil is distributed in all the engine its level may go less than the measuring sensor level. Try to put some more (not much) oil. 
If this will not help your measuring sensor may be bad and switch off when the oil gets hot.

does it have original oil in it? if so they use a lighter weight oil than recommended for engine break in. change your oil and filter and see if that doesnt solve it.could be thinning out loosing pressure when hot. i have seen this beofore.. :)