Most misfiring faults are electrical.
Since you have changed plugs and with 300KKs on the engine I would be looking at distibutor cap, leads and rotor button - all expensive items

If you have a multimeter you can check the leads. They should read 1000ohms end to end +/- 50 - 100 ohms. Check the distributor cap for cracks or moisture. (With all the wet weather you had in Warrnambool over the last couple of days (could be having an effect).

There is a very good independent M.B. expert in Geelong - I'll find out his name and P.M. you.