|The first thing I would do is check to ensure you have the most recent and up-to-date firmware. If your router already has the most recent, or downloading and flashing the firmware doesn't fix the issue then try recycling the power on the router.|
Simply unplug the power adapter, count to 10 slowly and plug it back in. Now try with more than one wireless client. If the issue doesn't repeat itself, you're good to go.
If the issue is still there, try resetting the unit to factory defaults and then reconfigure it. If the issue is still not gone, you may have a faulty router.