SONY TY voltage regulation problem

I have SONY KV-G 21 type 21 inch color TV, which was bought by my father 10 years ago. It works well for us more than, with small repairs. But from month ago, it had ugly fault. Sometimes, the image reduces from horizontal and stretch vertical and sound get stuck.

Unfortunately, when I remove the cover and try to test it...... Oh fault is not there and TV works fine !!!! What a headache. But, after two or three days, same story comes again..... OOOOHHHH..

In last Sunday, again I remove the back cover, and test the TV. I detect some sudden voltage drop form both 15V and 115V power supply outputs. According to the power supply design, 115V line is fully regulated, but it was 108V at that time !!!

Ohhhh.... This may happen with over loading, but my multi meter reading shows it's not. Then fault should be either primary side of the SMPS or in the secondary side.. The TV designers use STR-S 6707 as off line SMPS.

I just check the voltage across the opto isolater PC123 from secondary side, it varies.... It must be, but cannot present voltage if the supply is less than 115 V. !!! Nothing to say, the fault is in the SE115 - error amp IC. I just replace it and all come normal :-)