Sounds like a router issue from your description. Im having wifi issues as well. Do you use mac filtering on your router? I do and since loading CM7 i can go to settings, about phone, status and write down my mac address. Then toggle wifi off and back on. Move one screen back and then back to status and my mac address has changed on the phone. Ive done this six times now and it always changes. I started with an sbf before loading CM7. The mac address shouldn't change but it does.
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I'm seeing this issue as well, not quite sure what to make of it at the moment and I don't want to keep updating my router with a new MAC addr each time I reboot the phone.
I haven't found a solution yet. I switched over to MIUI just to see what all the fuss was about and found that it has the same problem. Of course it is CM based. Google'd the issue and it seems to be a common problem with CM7. I think most people believe it to be a problem with their router even though it is not. Their is also a widespread issue with CM7 users having the same MAC address on every device. This was on a different phone model though. The MAC should never change and it should be unique to every device.
I ran another SBF to make sure it wasn't an issue with my phone. Works fine on stock. Then loaded the original CM7 file and had the same issue come back. Loaded the latest nightly and still the same issue. I suspect everyone has this issue and they just don't use MAC filtering so they don't see it.