As always the trusty skylines community has helped resolve my missfire issue..
Symptom:
Got a service completed, engine bay cleaned and car started to develop a missfire under load through the rev range starting at 2,500 - 3,500 then 4,500 - 5,500.. Sounds like a machine gun popping..
Swapped out spark plugs which appeared to be quite old and coil pack completely failed when placed back.
Resolution:
Looked at a number of posts and ended up buying a set of 6 yellowjackets rather then splitfires from Performance Wise, arrived in 3 days - great service from Paul..
Put them in, car is performing as normal, no missfire!
Note: Car would idle fine and would be ok when gradually coming onto boost, only when wide open throttle and under load would it missfire..
http://www.performance-wise.com