Can you post your line stats from your router
Click on Status at the top of the main screen, then DSL Connection.
If you are prompted to login, the username is admin and the password is admin.
The fact that your profile is 500K suggests your line has recently dropped and re-connected at a lower speed. It usually takes 3 days for your profile to increase in line with the connection speed so you should get the higher speed back eventually.
You say you are connecting through an extension. If you have the new style NTE5 master socket you could improve things by removing the ring wire from the rear of the removeable faceplate on your master socket. This guide is helpful click here If at all possible connecting through the master socket is the way to get the best from your line.