We have Comcast, 75 gig service (I think — it’s supposed to be sorta fast). We have had numerous periods where it slows to a crawl, stalls, or just plain stops. We’ve tried to troubleshoot this, even buying our own Netgear dual band cable modem (C3700-100NAS). What tends to correct it is if I shut of my iMac 2007’s WiFi, the other devices and computers work fine. This affects the iMac when it is on, too. (When it’s the ONLY thing using wifi the problem rarely seems to occur.) Then, when I turn it back on, it’s either fine or reverts back to unusable. It’s hard to predict. There is no rhyme or reason. (It could be my wife's Macbook Air 2013). This is just normal web surfing and email. No video streams or torrents or bandwidth killers. Network Utility isn’t showing any clues. Our password is secure (and recently changed). We have two bands (SSIDs), and switching from one to another usually doesn’t help. My guess is that somehow devices are conflicting. How, I don’t know. That it can be fine for a while and then suddenly happen is weird, too. Both Macs are running Yosemite (10.10.5) with plenty of RAM and HD space. Any suggestions on what to check?