I had this issue once on a 2800 router acting as their firewall. The firewall is having trouble dealing with an excessive amount of fragmented packets. We tried upgrading, but it did not help. ISP said it was not on their side, but eventually the customer saw their truck down at the corner working on something, after that there were no more fragmented packets and no more downloading issues. So I would have a call with your ISP as well, unless you can download large files internally.