Ok. Just like the rest of the guys say...ignition- points AND wiring. I had a similiar problem like this as well, because the main terminal wasn't completley connected to one of the wires, and the main engine ground wasn't completely grounded to it's connector, just partially, so the spark was WEAK even with a new plug and would run very slow and flood/die/foul verrrrrry easily. check the wiring system 100% out.
-andy