Fire problem ... maybe

Hi,
so it's a stock '72 318 2bbl :sad2: ... I got it out lastly, start well, idle smoothly, rev well (up to 2500 rpm) when cold... Going for a little ride, to test somethings, about 5 minutes, then the idle was a bite rougher, a little bite of black and blue smoke when revved (2500-3000 rpm). If I raised the rpm from idle slowly to 2000-2500rpm, it sounds like if it misfiring (like if it's not running on all cylinders).
After that, I stopped the car, rested for maybe 20-30 minutes, then started it. Idle well, revved well...

I changed the cap distributor, rotor and plug wire (Accel 8mm), spark plugs have less then an hour of running.

What can cause that misfiring? The coil (most probably the original 34 years old)? The electronic ignition box?

Thanks for your help...