smb.conf:
[global]
        dos charset = cp866
        unix charset = cp1251
        workgroup = MKK
        netbios aliases = nex
        server string =
        map to guest = Bad User
        null passwords = Yes
        log level = 3
        log file = /var/log/samba/%m.log
        max log size = 1500
        unix extensions = No
        socket options = TCP_NODELAY SO_RCVBUF=8192 SO_SNDBUF=8192
        load printers = No
        disable spoolss = Yes
        os level = 255
        preferred master = Yes
        domain master = Yes
        dns proxy = No
        wins support = Yes

Master browsing can be intercepted by other hosts with multiply adapters. I think they became Master Browser in their own network and send annouchments to my master browser and confuse Samba. When it happens, all hosts in workgroup are dissapear with the exception of my master browser and intercepter. 


root@server:/var/log/samba# nmblookup -M MKK
added interface ip=192.168.0.5 bcast=192.168.15.255 nmask=255.255.240.0
Socket opened.
querying MKK on 192.168.15.255
Got a positive name query response from 192.168.4.28 ( 172.16.0.1 169.254.15.50 )
Got a positive name query response from 192.168.7.47 ( 192.168.30.1 192.168.147.1 )
Got a positive name query response from 192.168.3.50 ( 192.168.13.72 )
Got a positive name query response from 192.168.0.5 ( 192.168.0.5 )
192.168.13.72 MKK<1d>
192.168.0.5 MKK<1d>
192.168.30.1 MKK<1d>
192.168.147.1 MKK<1d>
172.16.0.1 MKK<1d>
169.254.15.50 MKK<1d>

In most cases the intercepter is one of 4.28 7.47 3.50 . Now my temporary workaround is to block these hosts in INPUT chain of iptables ;(.