No name resolve after IP-Change

[SWus]

Hi,

I have Bind (9.2.3) Server installed on Debian Linux (2.4.18 Kernel)
and SuSE Linux 9.1 (2.6.5 Kernel). Bind-Servers listen on local
interfaces and resolving local names and IPs and it works fine.
For external names the bind forwards to my ISPs DNS-Servers. To access
Internet Bindservers use a NAT-Router based on Debian Linux (2.4.26
Kernel) "iptables" using a DSL dial-up connection.

NOW the problem

After reinitiate dial-up connection the external name resolution fails
with

bash:nslookup

##
#> test4.de
#;; connection timed out; no servers could be reached
#>
##

or

##
#> test4.de
#;; Warning: ID mismatch: expected ID 18389, got 8398
#;; Warning: ID mismatch: expected ID 18389, got 8398
#;; Warning: ID mismatch: expected ID 18389, got 55750
#;; Warning: ID mismatch: expected ID 18389, got 55750
#Server:         192.168.0.8
#Address:        192.168.0.8#53
#
#** server can't find test4.de: SERVFAIL
##

The only way to solve the problem is a manual restart of the
nameservers.


I've sniffed the DNS-traffic on the router and recognized that
DNS-requests goes to the forwarders but not response returns.


Has anyone ever seen such a problem or a suggestion how to proceed?


THANKS   

SWus