If its happening every 30 minutes, I'd hardwire into the modem and run it direct for a period to make sure it isn't a modem issue.
You can also try reinstalling the firmware to the router, factory resetting, and then re-installing it. Reason I say re-install the firmware is a reset only clears out old settings. If something was corrupted it in, a firmware reinstall is what's needed.
Also, its tough to tell you what to buy when you gave us nothing to go off.
