Perfect wifi reception, IP-parameters correct, but doesn't work

Question

Hi all,
In my house I have several computers connecting to the internet through a Belkin N1 wireless router. There usually are 2 macbooks and 1 mac mini (c2d 2.0GHz), all now running Snow Leopard. Since the mini doesn't support N-networking, I've disabled that in the router. The macbooks connect perfectly, no problem there. The mac mini is another story.
When I bought it it ran Leopard, and contrary to the Macbooks it usually connected with a measly 2 bars in the wifi-indicator. But it worked. Since I upgraded it to Snow Leopard, it has perfect 4 bar reception all of the time, but it doesn't actually work. 90% of the time it is totally isolated from the network, despite showing perfect reception and all tcp/ip parameters configured correctly. If I monitor the connection by pinging the router, I see it works for a few seconds, and then most of the ping packets fail with either a timeout or a 'host is down'. It's not just the router, the mac mini can't reach the macbooks either. It appears that switching airport off and on helps, but mostly just for a few seconds, if I'm lucky it'll work for roughly half an hour.
Changing from dhcp to manual configuration makes no difference. The connection always works long enough for dhcp to retrieve the network parameters.
I'm pretty experienced with network stuff, but a system that indicates that all is well while it doesn't actually work, doesn't really give me any starting leads. What can I do?

Answer 1

Well, iStumbler answered some questions and created some new ones: Wifi reception is not erratic at all, it's constantly terrible, a stable 20%. All that time the menubar indicator happily reports 4 bars. Still, while both indicators show no variation at all, the actual network functionality will always go from perfect to nothing at all, a few seconds after switching on airport. Switching airport off and on again consistently helps for getting a few seconds of connectivity.
One afternoon, the indicator in the menubar correctly indicated the poor reception. Can you guess what happened? Yep, the network functioned properly for hours.
So it seems I have two issues to deal with; one is the poor reception that only the mini seems to suffer from (now there are some net interference issues in that corner of the room too, so I have some leads to follow there), and a bug in the Snow Leopard airport driver that at least makes it indicate a totally wrong reception quality, but probably also messes up the actual communication (since it should work with the poor reception, and it did until I upgraded from 10.5 to 10.6). Come to think of it, when installing the Snow Leopard upgrade, I had no problems downloading the huge amounts of online updates. So it might actually be a bug recently introduced.
The search goes on.. unfortunately without having internet radio on the hifi system while searching