Most of the time a W4 would have an H4 mag manufactured by International Harvester. By your description of running then cutting out with an eventual stall, I would be looking at the condenser and coil. An early indication for either one could be failure once it gets hot. If youíre unskilled with testing start by replacing the condenser as itís much cheaper. If that doesnít solve your problem try a coil.
Iíve rebuilt countless numbers of H4ís, they are quite simple. You also stated that you replaced your plugs. What brand? Itís been my experience that AC plugs work better than Champion in H4 mags.
If you donít know what youíre doing or if in doubt, send it out.