There's nothing more annoying than a router signal that's constantly fluctuating and cutting off your internet connection. It's always at that crucial moment when you're saving something or sending an e-mail that the darned signal goes and you're left in limbo until it reconnects again. So why do router signals fluctuate so much?
The main factor is quite simply, distance. The further away you are from your router, the more problems your wireless card will have holding onto the connection. The range of your router very much depends on the model, the specific 802.11 protocol it uses and whether there are any major obstacles between it and your PC. According to About.com, the average range of a router using the 802.11a, 802.11b and 802.11g protocols (the three most common) is 150 ft (46m) indoors and 300ft (82m) outdoors. However, as they point out, there are differences between the protocols and the way they react to different obstacles:
Obstructions in home such as brick walls and metal frames or siding greatly can reduce the range of a Wi-Fi LAN by 25% or more. Because 802.11a employs a higher signalling frequency than 802.11b/g, 802.11a is most susceptible to obstructions. Interference from microwave ovens and other equipment also affects range. 802.11b and 802.11g are both susceptible to these.
The problem is that the routers that Internet Service Providers give you when you sign up are invariably quite cheap and not very powerful. There's no way that my router has a range of almost 50m - I struggle to maintain a connection when any further away than less than half of that. But the weirdest thing of all is, even when the router is next to my laptop, the signal still fluctuates wildly although at least I suffer less broken connections. The answer I've found is to use a better router. Linksys and D-Link are known for their signal strength and if you're looking for something really powerful (and expensive) then check this out.
If changing the position and model of your router haven't helped, then it's most likely that the wireless card on your laptop or PC is at fault. Before throwing it out though, try testing your wireless router with an external wireless card. This plugs into your PCMCIA slot and will prove whether it's your machine that's at fault. It's better to use a PCMCIA card rather than one that plugs into your USB ports because they are generally better at picking up signals.
If none of the above have helped, then Microsoft have a good 10 point plan for improving your wireless network including a few extra tips such as replacing your router's antenna, adding a wireless repeater, changing your wireless channel and updating your router firmware.