About the intermittent problem of the network

Our company's internal local area network access to the external network will appear intermittent, when the network is disconnected, the ping router is connected, and the packet is not lost. The disconnection time is about 1 minute and 45 seconds each time, the maximum is 3 minutes, the continuous access time varies from a few minutes to dozens of minutes, the maximum is no more than two hours. When I turned off all the networks below the router, a separate computer kept on the network. Through this method, I checked each building and found that as long as there was a building connection on the 3rd, 4th, 5th, 6th, 7th, 8th and 9th floor, the network would be cut off. But through on-the-spot inspection, no loop phenomenon was found. I have had a headache for a long time. Which god gave me a solution? thank you very much! I am a novice in network management!