AirPort Express Wifi Speeds Fluctuate Vastly

I just purchased an AirPort Express 2013 (A1392), and it is hooked into an Arris TG862G/CT provided by Comcast. The wireless router function of the Arris is absolutely worthless (can't get a solid connection from more than ~45' away), so I decided to purchase the AirPort Extreme so I could get decent wifi in the living room of my apartment.

The router is currently connected to the modem via an Ethernet cable, and the modem is still broadcasting it's wifi network, since when Comcast attempted to switch it to "bridge mode," it simply torpedoed the entire internet connection - Ethernet and all. I ended up having them leave the wifi function on, since it was the only way I could get the router to work. I realize this causes interference. I set the Comcast modem/router to Channel 11, and set the AE to Channel 6 for 2.4ghz and left it at automatic for 5ghz.

In Airport Utility on my Mac, the Router mode for the AE is set to "off" and it's set to Bridge mode. As I understand this, it shuffles DHCP and NAT responsibilities to the modem/router in this case, but everything else stays the same.

The issue is, that similar to the performance of the Comcast modem/router, the speed of my connection varies dramatically, for seemingly no reason. I've done multiple speed tests sitting in my living room, using my Mac, iPhone, and iPad. At one point, all reported a top speed of around 40mbps down, which was fantastic. However, after I initiated a large download (PS3 firmware update), the connection slowed to a crawl. Figuring I had just tanked my bandwidth via the download, I cancelled the download. To be extra sure the download had completely stopped, I even disconnected the PS3 from power. I then performed speed tests on all my devices (Mac, iPhone, iPad), and got some seriously awful speeds. At their worst they hit .5 mbps, and at best they hit 2.5mbps. I tried all iOS troubleshooting steps for wifi (toggle wifi, airplane mode), and nothing seemed to work. Did the saem for my MacBook. However, after forgetting the network then adding it again, speeds shot right back up.

What's weird about the above scenario is that my wifi speeds were reduced by almost exactly 20mbps. I took my iPhone from my living room where I was now getting .5 - 2.5 mbps (and previously 36-40 mbps) into the office where my router was. Here, I now got about 20 mbps. Earlier (when I was getting fast speeds in the living room), I was getting 56 mbps or higher in this room. After removing and adding the network back to my iPhone, I then got the original speeds.

Can anyone explain this erratic behavior? I feel confident in isolating my devices out of the picture. What's strange is the Arris/Comcast router would exhibit similar behavior - sometimes the connection would be "okay" (I had tested as high as 20mbps in the living room, but rarely), but it would randomly drop down to <2mbps. I get a steady connection from all Ethernet devices. I've stress tested it, and it usually never drops below 56mbps.