I have the exact same issue with different computers here in the US when I connect to my Slingbox in the UK.

 Here is a rundown of the story from February on.

My Fujitsu Tablet PC runs Vista  Business.Streamed at 400kbs until Service Pack Upgrade in April. It now starts at 350 or so and then throttles back to 190 kbs.

4 Year Old Dell D505 running XP Professional used to stream at 400 kbs but only when connected via 802.11 Wireless. If it was connected directly to router via ethernet it would always throttle back to 190 kbs (This makes no sense at all to me as it is counter to what you would expect). Since June it now only streams at 190 kbs regardless of connection. I disabled Windows upgrades back in May on this Laptop so I have no idea what has happened.

My wife's Gateway running Vista Home always connects at 400 kbs and maintains it. Note: as yet this has never gone to Service Pack 1 Upgrade.

Really old Sony Vaio running XP Home and USB Wireless Adapter always streams at 400 kbs.

They all stream fine when connected to my LAN based Slingbox, it is just the one back in the UK which causes the problems.

I don't believe it is an ISP issue since some computers stream fine.