This sounds to me exactly like a classic switchmode power supply problem, the protection circuitry in the power supply, has sensed a severe potential problem, and is shutting down to avoid a fire, explosion, or both. This set had developed a problem, that is so severe that the set must be shut down, This set should be removed from the Power, and in my professional recommendation, you should get a "Quote" for repair, from, an, Approved Service Center, this point is very important for a good result, Always contact, their local, "Head Office" Now, when you get the information you want, then you can decide as to repair of replacement. The reason we want an approved agent, is that way, all the engineers are trained on their own brand sets, they have seen almost all faults, and will get the job done quicker, and for less cost, and usually have parts on hand, also sometimes, there has been a service bulletin in issued and the repair may be subsided or even free. There are many, many, many reasons that display this symptom, one must work through the troubleshooting steps to isolate and identify, the Cause and repair that and any damage done.
below is a link for a service manual, should you require one...http://www.schematics.me/purchase.php?id=Magnavox+26MD357B/37+Service+Manual+/+Schematics