Router latency issue

Not sure if this is the right forum to post this.
Anyhow…
I have a router that when I ping it, the TIME parameter shows huge delay (at times above 5000ms). I spoke to ISP and they said that it may be the virus crawling the subnet. I tried to unplug the router from the subnet and the TIME goes back to normal (less than 100ms). Is ISP right? What can I use (freeware) to test the subnet for abnormal node activity? Any feedback is greatly appreciated. Thanks.