[Ed Wright]

Alan said: "hook an undampened vacuum gauge to a manifold vacuum source, and check the valve springs."

Exactly what I was thinking. Found a lot of miss-fires other shops didn't seem to be able to find on my big old Sun Engine Analyzer bu using it's big, un-dampened vacuum gauge.
Invaluable tool if you learn how to use it. You can even isolate the offending cylinder with it. Just put a short piece of vacuum hose between each plug wire and the cap. Using a long screw driver with a jumper wire grounding it to a good ground, touch the short pieces of vacuum hose to ground the spark for each cylinder. If it's a cam or valve or spring problem the gauge will smooth out a lot when you ground the offending cylinder. It will deflect some for any miss-fire, but much more with a leaky valve.