HS Internet connection much slower after registry/adapter tweaks?

Question

Recently (according to speedtest.net log-3 days ago) my HS Internet connection (ATT Uverse 12Mbps down plan) slowed down dramatically after I attempted tweaking registry and adapter settings in Win7 to increase consistency in network speed.

I'm actually very capable with computer software and hardware (I build rigs), but I lack experience with the windows registry as I've rarely needed to modify things beyond simple adjustments in values/adding new items. With regards to adapter tweaks, nothing I changed should have any significant effect on speed.

Quite simply, I disabled all "green-mode" low power consumption settings, changed the duplex settings, and tinkered with some offload stuff.

Besides, I've tried resetting EVERYTHING, including adapter settings (by reinstalling drivers), using PCRefresh to reset windows service settings to defaults, flushed DNS settings and reset TCP/IP settings through Command Prompt, cleaning out registry values, reset network related registry items with TCP Optimizer. I even tried restoring windows, but to no avail.

I've determined it is not my ISP that is the problem, since every other computer in the house gets a steady 11.5Mbps down and 1.5Mbps up.

I am using an 802.11n Airport Express as my network receiver (does not do any broadcasting) which is connected to my MOBO's built in LAN adapter via ethernet. In case you're unaware, Airport Express is a very powerful receiver, and I've tried resetting all settings on it, but it hasn't changed a thing. I will link my old speed test result and my new (bad) ones, but textually, my old speeds were ~11.6Mbps down, ~1.5Mbps up. Now I am receiving anywhere from .4Mbps-4.9Mbps, but generally sitting around 2.

Link (old/good results): http://speedtest.net/result/2387614224.png

Link (new/bad results): http://speedtest.net/result/2439055663.png

So I guess my question is:

• Any suggestions on fixes I haven't tried?
• Any ideas specifically as to what could be the cause?
• I'm getting desperate and am seriously debating fresh installing windows, which would be such a pain. I have a lot of stuff on my computer, and since registry is likely at fault, I can't just completely restore, but reinstalling all those programs one by one would be awful. If I were to go this route, I'd download all of the installers for my apps onto an external drive beforehand to limit down time, but downloading those installers/full programs would take forever on this slow connection. Plus, what if I were to fresh install only to find that it was a hardware/connection issue that was the problem. It would be a pain to bring my custom rig downstairs to put my receiver right next to the router, but I could do it if you think this would prove whether it was a signal integrity issue. I must tell you though that my signal strength is "Excellent" so I doubt this could be the problem.
• When my connection is functioning properly, it's really great, given that I pay for 12Mbps and get 11.63. But often, even before these issues arose, by connection was very unstable and inconsistent, and it appeared to perform if anything, better during peak hours which makes no sense. I used to find that resetting router/adapter/receiver in certain orders, staggered by certain amounts of time would get things running fine again, but this should not be necessary, nor has it worked to solve the current problem. So any tips as to making my network less finicky?
• Finally, should I call up my ISP for help?