resolver problem

[exec]

It seems I have a problem with resolver.

ifconfig -a shows:

xl0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
         options=3<RXCSUM,TXCSUM>
         inet 162.x.y.z netmask 0xffffff00 broadcast 162.x.y.255
         inet6 fe80::250:4ff:fe0a:f6f0%xl0 prefixlen 64 scopeid 0x1
         ether 00:50:04:0a:f6:f0
         media: Ethernet autoselect (100baseTX <full-duplex>)
         status: active


/etc/resolv.conf is right:

domain my.uni.org
nameserver xx.xx.xx.xx
nameserver yy.yy.yy.yy
nameserver zz.zz.zz.zz

All nameservers are working properly on every other machine. I deleted 
host.conf file (which was also right) and nothing changes.

Commands host, nslookup, and dig all work very well for both internal an
external sites. But ping for example fails:

cannot resolve www.xxx.yy: Unknown host

where www.xxx.yy is any machine name except localhost (which is taken 
from /etc/hosts).

When using IPs everything works fine of course.

I'm also running a sniffer which displays all in/out packets to my 1st
NS when I run host, nslookup, or dig. But all others (ping, 
traceroute,...) fail without even asking the NS (ie. the sniffer doesn't 
show a single packet!).

I wonder if anyone has had similar problems and how and if they were solved?