Hi Geoff

I havn't had time to read all the other posts so forgive me if I am repeating anything.

I have done a couple of these now that have had cracked and burned out exhaust valves.

There is a gentle miss fire until the ecu decides the particular cylinder is not contributing enough power and cuts the injector to that cylinder leaving a definate misfire.

You need to do a compression check.

A cylinder leakage test will normally show the problem up.

Also if you put the offending cylinders piston to BDC and feed a boroscope down the plug hole with a piece of cotton tied to the end of the camera. You can pull the cotton and face the camera upwards onto the valves and have a good inspection of the valves.

If all these tests pass I would preferably scope the ignition and injectors.
Or, swap the offending cylinders injector with another cylinder. Same with the coil pack and plug and see if the code moves cylinders.

hth
James