with the rev counter dropping sounds more like an electrical fault than fuel...especially with the backfire. maybe a fault on the dizzy/low tension side of things as the rev counter is dropping....try wiggling /touching the low tension wires to the coil and dizzy when the engine is running to try and make the fault happen again....then remeber what wires you touched and look towards changing them.

also check all the contacts/connectors of wires for any of the green mank/corrosion that has a habbit of growing

good luck.