[boxhead]

Has this engine been running ok in the past and just started to play up?
A little previous history could help eliminate a few possabilties, (or open a heap of new ones).
Does the car miss when under load?
Are the plug gaps correct? I have seen engines with new plugs but wrong gap that missed only at idle.
The gaps i have found wrong have been too small.
I have also seen intake gaskets break away and end up inside the intake partially blocking intake runner.
Do as Craig (750hp) says and check the valves move when turning engine over by hand (worn lobe)
Move number 1 plug into another cylinder, does fault go with the plug to the new cylinder?
Let me know a bit of history on the engine.
Keep us informed