Ignition problem?

My #3 cylinder has always run a little cold
on the right mag only. I fixed what I thought
was an intake leak 20 hours ago and that helped,
or at least seemed like it, but today it became
clear that #3 was not firing at all on the right
mag. To make a very long story a little shorter,
the problem travels with the ignition lead. If I
switch top and bottom leads on #3, it still goes
cold on the right mag, which tells me that it's
not the plug. The harness checks fine on a lead
tester, and when I finally broke down and pulled
the mag I was able to 'spark out' all four leads
against the mag case. I was expecting either the
#3 lead would be bad or it wouldn't spark, but it
looks like that's not the case.

Anybody ever see anything like this? Anyone got
any suggestions?

Note that this is *not* the cylinder I replaced.
On both mags the engine is noticably smoother
than before the new cylinder.

Details:
Lyc O-320D
Slick mags
The mags, harness, and plugs are about 6 months old
Good compression check

Dave '3-2-1-pffffft' Hyde
RV-4 in flight test, EAA tech counselor