swapped in a good working Disty....3 plugs firing steady..but #1 is randomly missing. Changed to good wire on that cylinder...still misfiring. Definately a random pattern. Will go 3-8 flashes of the timing light just fine, then miss 1 or 2 (OR 3) and then be ok for 3-8 firings. Have not pulled that plug, since all are pretty new. Realize a bad plug might do this....but what else do we look for?

cap & rotor

And if the cap doesn`t work,try a compression check.The timing light isn`t really a reliable indicator of distributor function.A cheapo inline spark tester would be better.If the plug is fouled or the compression pressure is too low,the voltage may not go high enough to trigger the timing light.

I once had trouble trying to time a well worn engine w/low compression (intermittent flashes)until I introduced a gap into the #1 plug lead sufficient to raise the voltage for reliable triggering of the light.

when we brought over the working disty, it came with a working cap and rotor. Gonna pull plugs and check compression.