since a few days we've an unperdiodicly happening probelm on our GW-Server:

From time to time the Server couldn't translate the domain-names (ping on
ip is successfull, ping to name fails, there are also MX failures on GWIA

After a reboot everything works well (mostly).

Our GW-Server: NetWare 6.5 SP6, GW 7.01, GWAVA4, ZEN7.01 and dns direct
from external dns-server.

We also checked our firewall rules. There is everything correct.

Could anyone give me an idea what is going wrong?
Or did this happen to anybody else in the past?