There could be a couple of reasons as to why you have this problem. The most likely issue is the interconnection between your Garmin 276C unit and the GPS. You should take a look at the back of the unit - the connector is the usual culprit.
Also, it could be possible that the cable got chafed somewhere, so check for that next. Lastly, deal with the GPS antenna itself. Try getting another Garmin GPS compatible with your unit and just simply plug into the Garmin 276, this should solve it. Otherwise your unit needs to be sent it to be looked at.