Network Monitoring (please HELP)

I am experiencing some issues where I work. I am the Network Administrator and we are runnng a 10/100 network with an external T1 connection. We constantly experience latency in our Internet connection and reduced performance on our own network. OurISP claims all is well on their side, they have run tests on our circuit and have determined that we only peak at 30% utilization on our T1 line. Can someone offer a suggestion as to how this can happen and perhaps a good network monitoring tool to detect the problem.