Check that your router is set to a channel that does not have a lot of traffic on it. If you are in an area where there are a lot of wireless networks, this can be very important for proper operation. The other possibility I can think of is that the antenna is faulty or there is some other interference in your building. You might also check that your router does not have a low power wireless transmit mode, not many do, but if it does, it's always possible that it's turned on.
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Check that your router is set to a channel that does not have a lot of traffic on it. If you are in an area where there are a lot of wireless networks, this can be very important for proper operation. The other possibility I can think of is that the antenna is faulty or there is some other interference in your building. You might also check that your router does not have a low power wireless transmit mode, not many do, but if it does, it's always possible that it's turned on.