I spent some time last week rearranging my network because I had to get my servers out of my den and into my office. This required running a cable line into my office and a network line out. I setup the servers in my closet and initially everything worked out fine. However, by the next day my Internet connection had degraded into a sorry excuse for broadband. Where I normally get speeds in the Gigabits, I was instead getting speeds in the kilobits. I checked the modem admin page and could see the Signal to Noise Ratio (SNR) was frequently falling below 30dB, which is not good for cable modems.
I assumed the cause was the extra 50' of coax I used running a line into my office, so I moved the modem and router into the entertainment unit on the opposite end of my network (in my living room). The connection was okay for awhile then plummeted again. I had run my modem and router in the living room before with no problems, so I was confused as to why I was having such problems. Being used to troubleshooting technology (you know, the stuff that's supposed to make our lives easier), I disconnected every other cable line in the house. I was hoping a TV or TiVo was generating noise that was upsetting the modem. The SNR still kept dipping.
To make a long story short (too late), I realized the modem did not like sitting on top of the router. In every other configuration, I kept them separated. It turned out that the modem was building up way too much heat, and it really hates heat. I moved the modem to a nice location far away from other heat sources and everything turned out fine (so far).