Time to seek for the vanagoners (vanagonians?) expertise...

What i thought initially to be a carburator problem in my '73 bus is
manifesting now as a spark/distributor problem. My bus seems to be
running in 3 cylinders, maybe 2. After checking the spark from the
distributor with an inductive light i noticed that cylinders 2 and 4
were getting a spark unevenly and once in a while... when taking the
wire plugs from cylinders 1 or 3 the engine dies, whereas if i take
the wire plugs from 2 or 4 (or both), the engine doesn't change revs
at all and keeps running as before. Now, if i hold #4 wire right next
to its distributor hole, the cylinder starts catching sparks and the
engine seems to make an effort in increasing revs...

The points seem to be set correctly (dwell ~50 and remains +-1 when
increasing revs). I thought it might have been a problem with the old
and filthy spark plug wires that the bus had when i bought it, but
this morning i replaced them with new ones and the problem is still
there. I also changed the distributor cap, but didn't help. I'm not
sure where to go from here... could it be a bad condenser? The
distributor is a 009 mechanical advance. Any ideas?

Thanks!

Jose A. Madriz 
'73 bus (weak Shaggy)