I'm having some problems with my Internet connection. I'm using a
DSL-Router configured as "router mode" (PPPoE LLC encapsulation -
and DHCP Server)
I start a connection and my ISP assigns my an IP address. Everything
goes great, I do some 'nslookup' and it returns the addresses
immediately. But after 6 minutes approx. Web browsing gets very slow
and nslookup doesn't work, just says:

** DNS request time out
** timeout was 2 seconds
** Can't find server name for address "DNS Server": Time out

It only works if I increase timeout to 3 sec. (timeout=3)

If I reset my connection and get a new IP address everything works fine
(full speed web browsing & nslookup returns the Ip address of any
domain), but after 6 minutes the problem reappears.

I will deeply appreciate any help.
Thanks in advance!