Internet slow, getting new router has only fixed it for a small amount of time

About a month and a half ago my internett (verizon fios, 75/75) got super slow. It would take minutes to load pages (if they even loaded at all), but they would also (very rarely) load at their normal speed sometimes. The verizon tech guy ended up replacing my router and it fixed the problem until a few days ago. Same thing started happening and the router got replaced today, but the fix only lasted about 10 hours this time, and it's back to being slow. I've tried 3 different speedtests (speedtest.net, google speedtest, and the verizon speedtest, which has one for the router and one for the device) and all of them are saying that i'm getting the speeds I'm signed up for, except for the device speedtest from verizon. That one usually either fails because it can't connect or gives me super low download speeds (e.g 0.16 mbps). This is happening on all my devices (desktop, laptop, and phone), so I'm wondering what the problem could be. I'll be calling the technician tomorrow morning to stop by and hopefully fix the problem permanently, but I'd like to fix the problem asap. I've already tried rebooting the router, rebooting the ont box from verizon, and flushing/renewing the dns.