i have the same problem, and its non of the above as i have changed them all. so far its had a 2nd hand throttle body, new HT leads, new plugs, dizzy cap, rotor arm etc, (plus all normal service items) basically everything that has been said here i have changed, im now pretty sure its the o2 sensor but i havent changed it yet, will have to order one soon.
there is no circuit board on the GA engines to solder, i didnt realise this until i came to swap them over but decided to swap it anyway and hope it was a sensor on the throttle body that was knackered, obviously it wasnt.
may aswell change the oil every 3 months imo, costs about £30ish with decent oil which is nothing compared to what you put in in petrol, worth the cash imo.
give it some abuse for a little bit, mines a complete pain sometimes but after a quick blast it improves alot, aslong as the engine is warm with decent oil to the correct level etc it will be fine, the only way to kill and engine is either to be very unlucky or neglectful, the odd trip to the redline isnt a bad thing aslong as its not too often.