Cannot access shared folder on Snow Leo MacMini from Lion MacMini

[wwwzap]

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These are the two systems involved:
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Host:
Mac Mini 2006 1,1
OS X 10.6.8 (Snow Leopard)
1.66GHz Core Duo; 2 GB 667MHz DDR2
Ethernet connected
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Client:
Mac Mini 2007 (1,2)
OS X 10.7.4 (Lion)
1.83GHz Core 2 Duo; 2.5 GB 667 DDR2
Connected via WiFi
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Router:
WRT120N (DHCP server enabled)
WPA2 Personal + AES
SPI Firewall filtering Anyonymous internet requests and IDENT (port 113)
Lease time: 1440 minutes
It has been reset, and re-set up, with no other changes in settings
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There are no Windows computers on this network, and the only other router is a DSL modem behind the WRT120N.
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Initially, the host system with fire sharing enabled (AFP) appears under "SHARED" in Finder of the client system (Lion 10.7.4), but it appears as "Kind: PC" (with the associated "PC" icon), even though the share is clearly listed as "AFP://192.168.x.x/", and when I try to connect in Finder to view shared folders, the request times out after 20 seconds or so, with an error that the host is not available or not connected. To fix this, I disable the SPI Firewall in the WRT120N, if it is enabled OR, I ENABLE THE FIREWALL IF IT IS DISABLED, and instantly the host system is listed as "Type: Mac Mini" (with the Mac Mini icon), and I can connect and view and modify shared folders from the client system for about three hours, and then the shared folders simply stop responding (no error is returned), the host Mac Mini is listed as a PC, and I have to repeat said step with the firewall in the router. To emphasize: it does not matter if I am enabling the firewall, or disabling it, just the change from one state to the other allows the systems to connect. I MUST have sharing work with doing this because the client computer is operated by a person who is computer illiterate.
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Here's what I've tried: SMB and FTP sharing is disabled on the host. After updating the operating systems on both computers as far as they can be updated, permissions were repaired, new sharing usernames were created, firewalls are disabled on host and client.
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What am I doing wrong? Why am I loosing the ability to connect or see the host properly after about 3 hours?
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Thanks

[wwwzap]

I think it has something to do with WPA2/AES key renewals.