I saw exactly this same thing.  I also couldn't reach
   gatekeeper.dec.com.  When I removed my static default route (so the
   only route was the one that pppd installed (with the defaultroute
   option) when it started), all the problems went away.  Dunno why.

What interface does your `static default route' point to?  If it
points to anything other than ppp0, then trying to send a packet to
any host while the PPP route is not present will cause a route through
the default route (and its associated interface) to be cached.  Try
using `route get his-hostname-here' when you don't seem to be getting
through, and see what it says.

Of course this is annoying, and one would expect the route cache to
change dynamically, but it doesn't appear to.  Someobody should fix
it.  <hint, hint>  B-)