First off, I'd do the following with the router.
- For wireless connections, change the channel on the router, I like channels 1, 6, and 11 in the US.
- Update the firmware to the latest version available on the manufacturer's site.
- Reset the router to factory defaults and reconfigure.
Many times these measures will resolve a lot of intermittent issues.
Next, I'd see if you simply have low signal strength, many times adding a wireless hi-gain antenna will help here.