I have tried everything from restarting router to plugging computer directly into modem.
Each computer on network receives same lag spikes so it is not an antivirus issue. However, I also disabled antivirus to check. Tried with every computer in house shut down and one connected directly to modem and still nothing. Speedtest will return results of 28 Mbps and then 10 sec later will drop to 1 Mbps. I called Comcast and they do not know what the issue is.
Router and modem are pretty old so I am going to replace them and see if anything changes. Other than that, I am completely out of ideas.