Throttling the upload rate is a good idea. Last year I had several months of "line quality" issues that had a similar impact.
The power at the street was "dirty" and subject to brown out, the cable modem was is an "almost failed" state for a while, and the node off of which I hang was in poor condition. I got to know the people at the cable and power companies far too well. The upshot was the power company replaced the transformer on the trunk line with one of twice the capacity of the former, I replaced the cable modem, and ISP upped the capacity on the node.
Performance is better but througput limitations still cause saturation symptoms from time to time.
Good luck.