Inbound bandwidth Spiking

I got a serious problem. Every so often my inbound bandwidth spikes out of control. Sometimes its for few minutes or a few hours. My ISP is charging me for everytime I go over my allocated bandwidth. I had them run a sniffer once and it was able to find the IP address on my end that was the culprit. I can’t keep having them do the sniffer when it spikes, because they are charging me and arm and a leg just to run that. Is there any good software I can get where I can do this myself thats free or inexpensive?