you mentioned your download speed, but as well important is the upload speed in the location where of your Slingbox Pro HD becaue often this is the bottleneck if you are watching remotely. Could you maybe make a speed test and post the results here?
I think a good site might be http://speedtest.net/ (try to select the most close by location for the test).
Most provider use two different speed for upload and download, but mostly just mention the download speed (it sells better because of a higher value than the upload speed) this is called then ADSL for Asynchron Digital Subscriber Line. Asynchron because the download speed is different from the upload speed.
Maybe you could post some more information about your Internet connection of the place where your Slingbox Pro HD is located.
Hope this helps!
The Download speed of 1Mb is irrelevant. The important bit is the update speed at your Slingbox location. As you were getting 500K I would guess you probalby had a 784K (DSL ?) upload speed. Maybe your ISP has dropped it to 350K-ish ?
You also have to remember that Internet speeds can chnage dramatically for time of day. I get exactly the same symptoms as you, but have figured out it happens when everyone in my village comes home from work and turns on their PCs. And a bad case 'stuttering' was traced to my daughter watching You-Tube in her bedroom and grabbing my bandwidth