What AV you using check this out :
I had what I think was exactly the same issue. When uploading content over HTTP, using either a web browser to YouTube or Dropbox client to Dropbox's Amazon EC2 servers, I experienced:
long ping times (1500-3000ms)
packet loss
massive corresponding net traffic usage (1MB file took about 100MB to upload)
I traced the problem to a security feature on ESET's NOD32 v4.0.467.0 for Windows 7. I can confirm that unchecking "Enable HTTP checking" in "Advanced Setup -> Web access protection -> HTTP, HTTPS" pretty much sorts the problem out. I've blogged about it here:
http://www.alexstanhope.com/blog/techno ... ping-timesSolution: Open NOD32 "Advanced setup" from the quicktray menu. Uncheck "Enable HTTP checking" in "Web access protection -> HTTP, HTTPS".
In summary this was a horrible problem and it took many hours to resolve. It all comes down to the integration between software components. A feature in ESET's NOD32, which was designed to protect me from viri ended up behaving like a virus and wasting GB of net traffic, conflicted with Windows 7 and was triggered by legitimate behaviour in Dropbox. No one party will take responsibility, though I wish ESET would.