When i was encountering this issue one of the things I noticed was that the missfire would only occur if the car was under heavy load. eg: throttle at 80%+. If i drove the car normally i would not notice an issue.
It never threw any error codes and the problem had me baffled for a long time. I replaced my plugs, fuel filter, air filter, cleaned maf, oil and filters... everything i could think of.
I then started speaking to a holden mechanic who suggested I replace the coils, in his words they would "need to be replaced eventually". So i bought a set from eBay with the premise I could always resell them if they didn't fix the problem. Didnt take long to change them all any hey presto, that was it.
By the way.. how many kms has your car done... the issue started occuring for me about 120,000kms.