This can depend on have far you live from the nearest exchange,how busy your area gets at certain times of the day these can slow down your speed.
However when downgrading or upgrading you need to reboot your cable modem.
Also update netork adaptors vis device manager.
Could you run this after rebooting your modem:
http://www.speedtest.net/
Could you post the results back here please.
You may also need to run a few virus scans just to be sure:
http://www.download.com/AVG-Anti-Vir...-10320142.html
http://www.download.com/Spybot-Searc...-10122137.html
Could you also open your web browser and click tools/internet options and delete temp files/history/cookies/
http://www.ccleaner.com/download/
I hope this helps.