What is wrong with the modem/router?

Okay, so I have Comcast with a decent router and a fine modem. These all worked fined for a really long time, but then when we moved the router/modem, it just started working horrible in multiple ways. When we moved it to a different part of the house, a person two rooms away just could not get a connection. When we moved it closer to that person, the person it used to be close to could not connect. We finally found a spot where it worked for everyone (middle of house directly in hallway). It would work OK most of the time, but would just randomly go to **** for no reason. We have had this problem all the time, and all we had to do was reset the router, but doing that would do nothing. It would just go back to normal after a couple minutes. Also, it wouldn't even say it disconnected. I would have full bars but the internet would just stop working. When I would try to diagnose the problem, it would tell me to reset the router, then the modem, and when neither of those would work, it would just tell me to contact my ISP. We've done that, but they didn't help whatsoever.

We also tried moving it back to its original spot, and it will work fine, but every 10-20 min it will just go to ****, stay that way for 1-5 min, then go back to normal without disconnecting.

It is the most annoying thing ever, and I just want to figure out if it is the modem or router to see what I need to buy. I remember the setup working perfectly for months, but it just doesn't.