How do I know if my internet connection problem is a result of my router or modem?

[Miro]

My router is Netgear n300 wnr2000.
My modem is Arris.
ISP is Comcast.

Problem: I have been having a jumpy internet connection for the past few days (sometimes it is as fast as it is supposed to be which is 25 mbps download and 5 mbps upload but sometimes it is as slow as 3mbps download and 0.4 mbps upload). I do not lose signal completely, the internet connection is always working, it's just the speed is very jumpy.

I called Comcast and they checked the signals over the phone and said I was getting good signals. I reset my modem and router and still had a jumpy signal. I left them both unplugged/powered off for three hours and still the signal is jumpy. Watching Netflix is not possible because the movies load every 10-15 seconds. What is going on?

If this helps, the lights on the modem are on ALL of the time and do not shut off when the signal gets jumpy. The lights on the router are also on, the router has been working perfectly for the last few years. I don't want to pay for a Comcast technician if it's the fault of the router because the Comcast technician is only free when it's the fault of their equipment. I also don't want to pay for a new router if it's the fault of Comcast equipment. How do I figure out which one of these is having the problem so that I know which one I need to spend money on?

Thanks.

[Wyatt]

To narrow the problem down a bit try plugging the modem directly into your computer and running on that for awhile to see if the problem persists. If all is well, that'll rule out the modem and single it down to your router as the cause. If the problem continues I would suggest swapping out the physical cables, both to the modem and any cables the router. If you have other devices running into the router, unplug those also just in case. Keep the one computer that you're using to test the connection plugged in by itself. If in the end you're still experiencing issues I think a technician may need to check the actual cable line running from the wall to the modem.