Modem Crashing

I can reliably crash my modem. All I have to do is run a speedtest from any computer or phone on my network and it will restart every time. It always hangs on upload, download speeds are fine (on ultimate plan). Is my modem dying? Or is something else going on? Problem didn't start till a couple weeks ago. Have restarted modem and routers, computers etc. Called and had modem reset. Still it does it.