Wireless network problem

I asked this quite a while ago, but no one seems to know what's wrong. I'll try again. Here goes:

I have a network of Macs at home, both wired and wireless. When one of the laptops have been in sleep mode, it takes forever to 'find' the network. Why?
Sometimes you can see the symbols of the other computers in Finder (shown as generic PC's) - in some cases they won't show at all. If you hit cmd-K, nothing happens.
It can take up to 30 minutes before you can access other computers on the network. This is unacceptable. One laptop is a MacBook Pro with Mac OS X 10.6 and the other one is a MacBook Air with Mac OS X 10.7
Please help! I need this fixed!

It would be interesting to know if you guys can access other computers immediately after sleep mode or if you have to wait, like I have to.

 

This has no effect, I'm afraid. I do have immediate internet access, that is not the problem. I can access internet, but not the other computers on my network.
20 - 30 minutes after start up I can see and access the other computers on the network... THAT'S my problem.

 

Yes, all laptops are affected. Also my son's Mac Book.
The network is my own, so there's no service provider to ask... I use a Cisco wireless access point connected to a D-Link router. The D-Link router is connected to a D-Link Net Defend Firewall which is connected to the internet. Pretty basic.

I work with Windows networks and I do VPN tunnels and such every day, so I think I have a pretty good understanding how networks work - but this problem is beyond me...

 

DHCP is distributed by the D-Link DFL-200 Firewall

 

I have tried disconnecting the cables to the two Mac Mini's and one iMac that are connected to my network. No difference. There are no other devices connected to the network.

Regarding Service Provider there are two kinds; ISP (Internet Service Provider) and NSP (Network Service Provider).
There is no problem with internet access what so ever. The problem is purely internal and the same problem occurs even if the connection to the fiber cable (to the ISP) has been disconnected.
There is no NSP since this is my private network in my house.

I did make a small error in my first post. There is no router between the Firewall (D-Link DFL-200) and the Liksys access point. The DFL-200 is both router and firewall and is also the DHCP.

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There is a 4 port Netgear switch between the DFL-200 and the Mac Minis/iMac.
Disconnecting this cable has no effect at all. The same problem is still there.
I'm considering replacing the Cisco Linksys for an access point from D-Link.

 

With just the router and computers, of course it works since this problem only affects WiFi computers.
Like I said before, with only firewall/router and WiFi access point and two laptops - the problem will occur...