When at a slow idel it runs OK at higher speeds it backfires and quits running. Also under a load it will not stay running it backfires and soumnds like it is going to explode

Hi,
Diverse things can cause this. One would be a partially sheared flywheel key. Another would be a partially blown head gasket. Still different would be something wrong with the valve train. And finally a failing armature that is having the builtin advance system failing. Inspection is in order and always start with the least expensive. The head gasket can be inspected with a flashlight shone between the fins to where the cylinder meets the head. The valve cover can be removed and the clearance checked, rocker arm pivot bolts inspected for loosening and pushrods for bending. All this with only time involved. They proceed to the key, and the armature would be last since it involves the most expense. All the while in the inspection, the wires for the ignition ground should also be inspected for bare spots.
Peace,
Clarke