This is a fault solution which can be checked and corrected by you assuming that there is no component failure. There is a high possibility that the components over a period of heating and cooling, tend to bring on a problem of dry soldering. So my advice is to check the set for some loose joints that have played on to give you these erratic shut downs or poor performance.
The best way to check for loose connections is to tap lightly when the set is powered on , If there is change while tapping then it seems that there are DRY solder joints. Remove main plug, the covers - with caution of high voltage, check for dry joints, use a good soldering iron to solder all suspected points.
Sometimes the fault occurs when the set warms up, if so you can simulate the heat using a hair dryer on the suspected areas, try with the power supply, distribution, Processor controller, driver controllers on the A/V related sections. Also the use a FREEZE-IT spray can simulate the components to cool and show up the fault.
Based on the above method you must be able to locate the area of fault and solder the sections. Sometimes large IC's- ones with more pins- tend to have drying in some pins and so cause faults related to the IC's protocol. Have a rough idea of looking into the areas to reduce the time of probing.
Check for tips to look into the SMPS unit with some of my tips:
http://www.fixya.com/support/r5826301-working_switch_mode_power_supplies
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