Getting strange behavior using vpnc to connect to a cisco vpn:

all works great when connected to a router (NAT)

when used in config without a router (common cable modem setup), it connects to the vpn, but does not pass "real" traffic
pings work, sometimes text on webpages (but not images), but cannot transfer files or run server side apps.
It looks like an mtu issue, but several fixes for that have no effect.

Also have tried :
upgrade vpnc - didn't work
downgrade vpnc (to the version on knoppix 3.3) - didn;t help

this appears to be a bug in the TUN driver in the kernel ?

a test with ubuntu 7.04 (feisty) live cd does not have the problem

has anyone seen this sort f problem or have any ideas?