if you are a cable suscriber your bandwith decreases if more customers are added to that pipeline but if you are ADSL subscriber bandwith decreases on the distance to the exchange.
Check the status of your exchange if co-op have one. Im with talktalk and they have an exchange status checker this shows the status of your connection, tells you if your exchange is live, speed of your connection and the area where your telephone exchange is. If they have moved you to another exchange that is further away this could be the reason you are getting slow connection. Also if you are running a router make sure all the lights are lit green at all times, if the adsl light turns amber, you are loosing internet connection and you need to ask your service provider to boost up your signal, but before you do that try to change your telephone splitters that seperates your phone and internet signals. Remember some companies block icmp packets for security, so you would get a unreachable reply back. Hope this helps.