|First, check to ensure you have the latest firmware installed on your unit. If you don't, upgrade it and see if this issue goes away.|
If you have the latest firmware, try a reset (ie: power it down, count to 10 and then power it up).
If that doesn't do the trick, try resetting the unit to factory defaults (ie: hold reset button in for 10 seconds and allow a couple minutes after for it to reboot) and set it up again for your network and then reboot all connected clients and see if that doesn't fix the issue.
If none of the above fix the problem, your router may be in need of replacing.