i've been having this same problem, also on a 2008 macbook for the last couple of weeks now, so it started around the same time as yours. i was convinced it was my internet connection, and had my provider come out a couple of times to check. then i discovered that other computers connected to my wifi had normal speeds - it is only my macbook that is slower than dialup. i've tried pretty much all the things you have, and nothing works consistently - though occasionally i can get back to normal speed, but only for a few minutes at a time. what can this be? dying airport card in the 2008 macbooks?
ive never had that issue, my internet always has been faster then anyothers on their pc our home, mind you wifi sometimes doesn't work on my mac, but i blame that on its age, and how ive treated the laptop.
i wouldn't know why its doing that, i mean i get slow internet sometimes, but when i get it slow so do all others on the same internet. i guess its like everything out there, there is always a lemon even with the best of products.
sorry to hwar about that