About Optoisolator IC
The function an Optoisolator IC in power supply is to send a signal back to the Power IC so that it can control the “On” time of the switching frequency. Because of this the output voltages at the secondary side always stable. Now, in certain power supplies you will notice that ther are more than one Optoisolator IC. The other Optoisolator IC could be part of the standby circuit or even for controlling the power ON/OFF in CRT Monitor. That means if the VGA signal cable did not plug into the computer CPU, the Monitor power supply will shutdown.
Assuming if one of the Optoisolator have problem, it could cause no power, power shutdown or even power blink. You must solder out the Optoisolator IC and test with your analogue meter. If the Optoisolator IC that control the power supply on/off have problem (usually open circuit in the phototransistor) you can simulate a test by soldering a jumper across the transistor pin. Of course this will defeat the purpose of the circuit. That means, the Monitor power supply will be always On even without plugging in the VGA cable to the CPU!
If the Optoisolator IC control the regulation have problem, then solder a jumper across the phototansistor pin will not have any affect to the power supply. In this case i suggest that you get a new Optosisolator IC and retest the power supply again.