I have had so much trouble with my wireless hookup that I am about to junk it. I will get it working, and it is OK for a week or so, then overnight it stops working. When I try to get it operating again, I sometimes have a lot of trouble. Could it be a defective router?
       -- Dave Yalden

It could be, but first let's eliminate other possible causes of the problem.

First, make sure you are using the latest drivers and firmware for your wireless card(s) and the latest firmware for the router.  If the problem continues, we need to start looking for either a pattern or something that can be reproduced that could be causing the problem.

Use your network card's site-survey function to see if there is another wireless access point nearby on the same or nearby channel.  Moving to an unused channel should help clear this up.  In North America, using channels 1,6 or 11 should help.  There is enough seperation in frequency between these channel  that this should help cure some if not most of the problem you are having if it is a nearby access point occasionally causing interference.

If you don't have the access point plugged into some type of surge protector, consider doing that now.  It is possible that some type of electrical interference that might be causing the access point to fail. When you can't get into the access point with the wireless card, see if you can get into the management interface over the wired network.  If you can, see about toggling the radio inside the access point on and off to see if it is shutting down for reason.  Look at the logging that the unit can generate for any possible errors there.

If your access point has directional antennas, point them where you need them - this can also help eliminate or reduce interference. A simple patch antenna can help direct the access point "footprint" to where you want it. This may take some experimentation to find which antenna may do the job for you.

If you are using encryption, try turning it off termporarily to see if that might be causing a problem.  I helped a company troubleshoot a problem where they were periodically losing the access point or getting such slow response as to make it unusable.  By process of elimination, we discovered that WPA in the particular access point had a glitch and was the cause of the problem.  The problem didnt show up with WEP.  In either case, make sure you turn off the broadcasting of the SSID of your access point and change the SSID to be something unique - not something that can identify your company or the location of the access point.