My guess would be check your ignition timing but like voguese39 said genuine cap, leads, rotor arm are essential!!
One other thought, you might have an induction leak possibly, although that might more often be associated with the engine starting but then stopping after a couple of seconds.
If it wasn't for the same problem on LPG I might also say check your fuel level but it sounds like a timing problem. If memory serves you want 5 or 6 degrees btdc. Do a search I have posted timing settings before.