Wireless works then stops working. What could be possible solutions?

My modem needs to be reset for wireless to work then it only works for about 15 minutes at a time. I have a desktop that is directly connected to internet and works fine. I tried to connect other devices such as my laptop, iphone, and Wii yet all have the same issue; wireless only works after resetting or power cycling the modem (which has the wireless router built in. Believe it's called a gateway modem) then stops working. The make of the modem is SMC Networks which Comcast sent me. I used to have a modem and a wireless router but I had the same issue so I'm thinking it has something to do with my desktop settings or modem settings.
I went into the modem's URL and set it to automatically find the best channel and still nothing. I also manually tried each channel and still nothing. I turned off my firewalls from desktop and no progress.
I don't know what else to do. Please Help!
Any suggestions are greatly appreciated! I'm not that technologically savvy so the more detail the better!

Sounds, to me, like the wireless portion of your modem provided by Comcast, may have a defective wireless portion.

The easiest answer to fix this, is to turn off the wireless portion of the modem, and connect an external wireless router to the modem from Comcast. I find that the Linksys/Cisco routers work best (I have one with Comcast), and should be very stable.

If you call Comcast at 800-266-2278, they should be able to tell you how to turn off the wireless router portion of the device, making it a simple modem. I would recommend the Linksys/Cisco E2500 router, as it gives you the most flexibility.

The setup of the Cisco/Linksys router on Comcast is really simple, All you need to do to set it up is to run through the configuration and save each page separately. Once done, you should be good to go. Don't forget to set your passwords (there are three for this) for the wireless access into the router.