distributor problem

I have a misfiring problem that I can't track down.  It HAS to be in the
distributor, cause if I put another in its OK.  Points look good, gap set, new
cap, wires about 10,000 miles old that check ok on a high tension lead tester.
The dizzy is a Mallory dual point, with less than 10,000 miles.  WTF could be
happening here??????  Could it be a condenser that is crossing over
intermittently?  I'm really stumped.  Anyone???

Geoff Branch
'74 Meejit "Yellow Peril"
'72 Innocenti 1300 Mini