[Thumper]

Long shot, but might be the fuel filter. Also, when you had the carb out did you check on the needle valve that the float opens? Could be some crud there.

The fact that you had it running great does suggest you had the jetting right.

It doesn't sound electrical but the spark plug may have gone out of resistence range. I know, these seem like unlikely things, but you already worked on jetting (which shouldn't be off), and even went to the trouble of checking the valves.

Back firing might suggest fuel blockage (either the filter or the float bowl not filling). Do you notice back firing when it acts up. Also, if you let it sit for a few minutes, does it run right all of a sudden?

Also, is the head-pipe right at the head getting too hot (indicates lean mixture as well).