Tom,
Get this TV to a shop IMMEDIATELY!!! What you are seeing a something called "shutdown".
You see, TVs can sometimes suffer from problems which, if left undiagnosed, can cause major-league irrepairable damage to its circuitry, the operator, or both. To keep this from happening most TVs are designed to automatically shut down if any of these problems are occuring. This sounds very much like what is happening here.
I hope that helps.
Graham's TV Service
West Columbia, South Carolina
Jim Lee- technician, May 2004