It could be a bad DTA, or maybe just a lousy signal to the DTA. Try going on-line and doing a self activation, if that doesn't do anything, try swapping it with another box (if you have one), Then it's off to Comcast to get a replacement..
If swapping out the DTA doesn't help, check all connectors and look for damaged coax cable. If you can't find the problem, have Comcast send a tech out to identify the cause and correct it.
If the tech finds the cause to be bad coax, splitters, or connections in your home you'll probably have to pay for the visit unless you have their Service Protection Plan (about $4/mo). If the trouble is due to a faulty Comcast box inside or anything outside, you shouldn't be charged.