Measure your 5V Rail. if it falls too far below 5V, it will be caught in the bootloop of doom. All capacitor related I bet.

Another thing is the opto-isolator. They get weak with age, especially if it was used a ton. Most likely it was as it came from a school..

As the LED gets weak in the opto-coupler, the regulation circuit has to increase its loop gain to keep the output voltage at its correct setting, eventually the feedback/regulation circuit will get too weak, and the gain will be maxed out/saturated and the voltage will remain low all the time no matter what you do.

Also, dont forget your line-input safety caps. If you feel as though you cant accomplish the task entirely on your own, There is also uniserver which is a member here that will do the service for you