I have a DCS FSF 75L fryer that cycles fine for a while then flame & pilot go out? Why

Sounds like a thermocouple problem. Might be a loose connection at the gas valve, or the pilot light being blown/going out.

Attach propane, light fire, cook.

The difference between Natural Gas Set Up and Propane Gas Set Up on Fryers are as follows:
Natural gas burner orifice has larger bore (#52) than Propane Gas burner orifice (#55 or #56).
Gas Valve's spring kit for natural gas has less tension (thinner) than Propane.
You have to order burner orifice ( the quantity depends on the number of burners your fryer has), pilot orifice and Gas Valve Spring Kit for Propane Gas if your fryer is set for Natural Gas or vice versa.

Regards.

Limit switch is last-ditch cutoff.

The thermostat that SETS the temp is where you should be looking.

Contact your gas utility. They are usually very helful and may even do the conversion.

You must first check to see if your pilot flame is good. If it is too weak, or not impinging properly on the thermopile, it will not stay lit. If the pilot flame is ok, the problem could be that the thermopile is weak. using a millivolt meter, test the voltage across the thermopile while the pilot is lit (you may need someone to hold the button down while you do this). You will also need to disconnect the thermopile's leads from the control while you do this. You should get about 750 millivolts across the thermopile while the flame is on it (let it heat for a while to stabilize). If your reading is too low, your thermopile may be shot and needs to be replaced. That is of course only if your flame is proper. That voltage is needed (or close) to keep the magnet open so it will stay lit when you let the button go.

Try 3wire.com

This sounds to me like the thermostat is not sensing and is reading a closed circuit all the time. You will probably need to replace the thermostat.

Thanks
Zerko

This is caused by a failing coil on the gas valve. You should replace the gas valve.