340 Not firing on all cylinders

This is what I have found to work for me in this very situation...

Make sure cap and rotor look good, then...
Use the aforementioned timing light and hook it to each individual wire while running, looking for consistent strobing of the light. Make note of good cylinders as you will swap the plug wire (and plug if necessary) into a suspect cylinder. This will enable you to rule out first the plug wire, and then the plug, as a cause. What you are doing is taking a KNOWN GOOD part and swapping for the suspect part.

Do not just throw new parts on it and expect a miracle. I can tell you from experience that new parts often times are bad right out of the box. In fact, I went through 3 new sets of 8 spark plugs (thats 24 plugs total) using this troubleshooting method to get one complete set of 8 that worked.

Failing this troubleshooting session will unfortunately point toward a mechanical problem as mentioned earlier.