Ok, I've finally gotten it working.  I spoke to a nice young man named Kumar
(at lest they aren't lying about it now), who although was confused by the
fact that I had made my own router did give me some help.  He instructed me
to put the adapter on a DMZ and disable Q Oh S (yes, he said it just like
that).  So from this I got that I should add a rule to my routers firewall
table to default pass any inbound traffic to the Vonage adapter (outbound
was already default passed).  Now this shouldn't have made a difference
since the only inbound connection attempts to the Vonage addapter should be
in the 10000-20000 range, but apparently my router was having trouble with
NATing to the adapter (not identifying that the inbound traffic was in
response to the outbound--because it was through UDP?).  When I set to pass
all traffic in the firewall, it cleared up the voice quality problems and
has been working without a hitch for a few days now.

Thanks for all your help,

Richard