Well. If it has worked fine until just recently it could be malware, something running in the background and taking up your network usage, your firewall/anti-virus, or it really could just be your isp oversold their services in your area. Obviously they would not tell you they have done that. You could try setting a static ip address or disabling Nagle's Algorithm.
I personally just disabled Nagle's Algorithm yesterday and while I saw I decrease in my overall ping, it did nothing prevent the fluctuations that happen due to my isp.
http://b.rideekulo.us/2009/09/windows-7-nagles-algorithm-and-gaming/
Follow that step by step and you should have no problem doing it.
As "Idewitt" mentioned it could just be the server you are using...