OK...don't give up yet...let's keep going.
First let's rule out the fuel system. Make sure that the needle ans seats are sealing well. Do you have the fuel pump wired to a toggle switch? If not, try a temporary switch....turn off the pump when it starts acting up, although this doesn't sound like a fuel problem, but you said that your plugs were sooty, so....
Next: Check all your plug wires, maybe they are arcing. Try this in a very dark place and look for the arcing.
Then, if that is OK, it has to be coil/ignition box......