As Kevin alluded, the 2.4 GHz frequency range is very crowded (congested) because it's an unlicensed range used in many wireless devices: TV, microwave, radio astronomy, mobile phones, wireless LAN, Bluetooth, ZigBee, GPS, 2-way radios, etc.
If your wireless LAN was stable 2 weeks ago, either you recently added something onto the 2.4 frequency or...because of its longer range...a neighbor recently added something. You probably just need to find a "cleaner" channel.
2.4 GHz is not just one channel. That "2.4" is actually divided into 14 channels (11 channels in North American). In the 2.4 notation, there are 2 more digits to further increment the frequency into the thousandths: 2.4xx. Frequency 2.4xx starts at 2.412 and ends at 2.484 (ends at 2.462 in North America). These thousandths of increments are called "channels."
You can find a cleaner (less congested) channel with a 3rd-party program. WirelessNetView is a good program to scan for a cleaner channel in your neighborhood; however, I'm sure other contributors may recommend better programs.
