I've gone over the tubing looking for leaks and it all seems patent.
I pulled everything apart, lubed and reconnected and it's looking good
(including the much-maligned quick-connector). Unfortunately, I
haven't had a chance to fly it to find out if the tubing was my
problem or if it's the instruments (but BOTH varios behaving badly the
same way seems to indicate that it's not the instruments).

On the plus side, one off my club's members is a licenced instrument
mechanic and he's said he'll take a look at my setup when we can get
together. Hopefully that'll get it fixed.

Now, is there some way that I can use this as an excuse to convince my
wife that the "mistress" needs a panel upgrade?