Could be many things, but sounds suspiciously like the problem I have been chasing on my 1700 x/flow for a while and have just diagnosed. in my case, the bolt holding the trigger wheel onto the crank had come loose, meaning that the trigger weel was stopping in different positions each time I turned the engine over (it is keyed into the crank pulley, but the key pin had sheared under the strain)
Worth checking?
Otherwise, I suggest
- checking that you've got a spark at the king lead and at each of the spark plugs
- changing coil, rotor arm, distributor cap, plugs (all cheap parts, and if they don't do the trick, worth keeping in as spares anyway!)
Best of luck Michael