[V6-12v] Bizarre Cooling System Problem

[Wilfried Link]

Here's a great brain teaser:

Cooling system functions perfectly but for small leaks
at four different hose connections and a moderate one
past the O-ring at the coolant pipe where it enters
the block behind the thermostat. Engine is from a
wrecking yard with 75,000 on it. Rest of car has
182,000. New upper radiator hose and said O-ring .
Cooling system flushed. Heater works perfectly. Hose
connections have been adressed by cleaning
connections, tightening, loosening, you name it.
Clamps are way tight, still leaking! O-ring replaced,
still leaking! Only thing I can think of is
overpressure, but cap was swapped and tested (15 psi),
no difference. Aaargh! Water pump was replaced two
weeks ago by 2Bennett Audimotive after it stranded me
in mountains by gushing coolant out of the freeze plug
at a horrendous rate - which sounds like blown seal,
right? Only old part that plays a role in this to my
mind is the radiator. However, even in hot weather 
the system works perfectly -- oil and water temps both
happily normal. This car is to take us 3000 miles into
Canada and back in a few weeks, and at this rate I'll
spend more on Pentosin than gas!   Any clues out
there? I'm about to put in a new AC compressor ($595 
A-Air Online Florida -- great manager, Keith-knows our
compressors very well), and just sank a ton into this
newer motor (2Bennett again), so I tire of high-dollar
fixes.
Thanks in advance,

Wil
1995 A6 Avant
Vacation Cruiser