AirPort problem: Losing connectivity at recurring intervalls!

I'm using an Apple Airport Extreme. I have a MacBook running Mac OS 10.5.6 and an Xbox 360 connected to my network.

Now, to the problem:
At seemingly random intervalls I lose almost all signal strength and my download/upload-speed goes down to almost zero, and stays so for about 3 minutes, then goes back to normal. My ping also gets quite high.
The intervalls are sometimes as short as 15 minutes between two "drops", and sometimes there's a whole day without anything weird happening.

It should be noted that I don't get logged off my wireless network with either the MacBook or the Xbox, but as the network is virtually unusable during the time it gives a similar effect.

It is very disturbing, as I get logged off of many internet-demanding applications (MSN and the like) on the computer, and kicked out of games on my Xbox. Also, websites can't load.
Both the MacBook and the Xbox 360 is connected wirelessly, but it doesn't get better when using an ethernet-cable.

So, does anyone know what could cause this issue, and how to fix it?

Wireless dropouts can be caused by various things; interference, security, and so on. But if you're experiencing the same difficulty when connected via ethernet, it either has to be hardware related or the ISP provided signal.

If you have another machine (even a PC) that you can connect to your internet output, that would give you a simple test of whether or not it's hardware caused or from your ISP.

Substituting another router in place of the Airport Extreme would also be a good test of the AE to see if it's operating normally. Sometimes a defective router can cause the same symptoms you described.

Regards.

Thanks,
I'll connect my computer directly to the modem (where the Airport usually is connected) via ethernet to try to sort it out then.