I've been running a Linksys WRT54G for many years now without issue. We have about 15 devices that are present on our network at various times, including Windows XP, 7, 8, OS X (Mountain Lion), Linux(es), 3 iPhones, 2 iPods, a Nintendo DSi, Nintendo 3DS, Apple TV, Roku, Raspberry Pi. Until recently, all was well...
About three weeks ago, I started to notice that streaming anything on my iPhone over our wi-fi was next to useless. Doing speed tests revealed that I was getting less than 1Mbps down and about the same up. From a Linux machine wired to the router I get 25Mbps down and about 4-5 up. Same story with OS X over wi-fi. Same story with Windows. But my iPhone's connectivity was horrible. So, I thought it was a problem with the iPhone. Nope. Same on two other iPhones. So, thinking that it's about time we upgraded to a modern router, I purchased a NetGear WNDR4500. It's a dual-band gigabit router, and more suited to our plethora of devices. The thing sucked big time. Going beyond the Amazon 4.5 star rating, I found that others had issues with it, so it went back to the store. In its place is an Asus RT-N66U. It's pretty awesome... except, I still have the same issue with all three iPhones. My iPod, incidentally, displays the exact same issue.
Trust me, I'm an IT professional. I've done various network resets on the iPhones and I'm sure there's no weird DNS issues at the root of this.
I'm starting to think that it's a problem with the cable MODEM (a Ubee DDM3513). Is it possible that it's dying and this is a symptom of constant drops in connectivity... that are somehow masked by 5GHz devices? It doesn't seem likely, but it resembles a similar issue I had with a MODEM a few years ago. The only other thing I can offer is that it seems to get better at random times, but it's bad more than it's okay. And the whole time, speedtest.net gives great results on desktops and laptops.