Fuel starvation or constipation
If the converter was clogged it would not take more than a few minutes or a heavy load, Get a non-contact thermometer from the parts house or Harbor Freight, with this you can check for hot spots, any restriction in the exaust will be hoter than the rest.
In the old days a bad coil would do this as well as an incorrect gas cap (someone puts a non-vented cap on vented cap type system..don't ask how I know this), no single coil and gas cap sealed.
If you have a clogged vent in the fuel system the tank will slowly get into a vacuum condition that even the fuel pump cannot overcome.
Test for this is to open the cap as soon as the motor stalls, if a lot of air rushes in and the engine starts right up you may be close.
You can also locate your fuel tank, see what it looks like before you drive, then see what it looks like after about an hour, if SMALLER then no vent.
Leave the cap off or slightly open and see if it works, the cap itself is a sealed device so the venting comes from a cannister under the hood, the hose could be pinched or plugged.
If you can get under the hood without a lot of work you could take the thermometer and measure everything under the hood after the engine is warmed up, modules, boxes, plugs, just about anything, make a drawing or take a photo and print it out, write the readings on the drawing.
As soon as it stalls retake the measurements and look for something a lot hotter or cooler than before, this will indicate something getting hot and failing, or loosing power and not working.