Assuming that your Computer is configured correctly and there is No ?kinky? settings that you did not mentioned.
Step One.
Connect the computer to the Router with a Wire.
Works well for the day it is the Wireless fault,
see step three.
Does Not Work Well with a wire it is either the Modem, or the Router.
Step two.
It is the Modem or the Router. Connect the computer directly to the Modem.
Does not work well while directly on the Modem, call your ISP.
Works well with the Modem, check the Router?s setting (MAC Authentication, IP lease, and The Keep Alive the Connection settings).
Step three, OMG it is the Wireless.
Put the Wireless Source next to the Router.
Does Not work well, go carefully over the Wireless setting, load Netstumbler and look for noise from other sources.
Works well in close proximity, then:
The best Method to Extend Wireless Coverage depends heavily on the Specific Environment.
Look at these pages they describe most of the options that are within the means of Entry Level users, and choose the one that fits you environment best.
Link to:
Extending the Distance of Entry Level Wireless Network.
Link to:
Wirelessly Bridging Home / Network.
Link to:
Hi Gain Antenna for Entry Level Wireless.
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