I experienced the exact same problems a few months ago. I messed about in the router settings and accidently altered my username that I connect to my ISP with. I didn't have it written down anywhere so I had to phone them up to get it. As soon as I entered it again my connection went back to normal and has been fine ever since.
I had previously tried resetting router etc but nothing worked. I too had a normal speed for downloads. I was about to buy a new router as I was sure that was the problem.
Sorry I am not much help but just to let you know you are not alone.
