Slaves not picking up when master is offline

[Andre]

I'm having a problem with my DNS-setup. I'm running 1 master and 2 slave nameservers. These servers are authoritative for about 20,000 domains which we registered for our clients.
The problem is; When the master server is offline for several (4+) hours, the slaves don't seem to pick this up properly. Several of our customers can't connect to their domains because of unresolving hostnames.

AFAIK the set-up for the domains is ok. Here's an example of such a domain in the masterserver.

$TTL 14400

@ IN SOA ns1.nameserver.net.  info.registrar.net.   (
                                         2007102401  ; serial
                                         10800  ; refresh
                                         3600  ; retry
                                         604800  ; expire
                                         86400)  ; min_ttl

domain.net.         IN NS ns1.nameserver.net.
domain.net.         IN NS ns2.nameserver.net.
domain.net.         IN NS ns3.nameserver.net.
mail.domain.net.    IN A *.*.*.*
www.domain.net.     IN A *.*.*.*
localhost.domain.net.       IN A 127.0.0.1
domain.net.         IN A *.*.*.*
ftp.domain.net.     IN A *.*.*.*
webmail.domain.net.         IN A *.*.*.*
*.domain.net.       IN CNAME cname.domain2.net.
domain.net.         IN  MX 30 mailserver.registrar.net.
domain.net.         IN TXT "v=spf1 a mx ~all"

- All domains in the masterserver are defined as 'type master' in named.conf
- All domains in de slaveservers are defined as 'type slave' and contain a reference to the master
- All 3 nameservers are listed correctly at our registrars, checked through whois
- zone-transfers and updates are working fine, so communication between master and slaves is ok

The only possible explanation for this problem that I can come up with is that the nameserver's hostnames (nameserver.net in this example) are also resolved by these servers, the nameservers are authoritative for their own hostname. Is this a problem? As far as I know the registrars keep a hostname as well as an IP-adress for these 3 nameservers, so this shouldn't be a big problem.

Does anyone have any idea what could be wrong with this setup?

Thanks a lot in advance!

Andre P.