Consider the possibility that the switcher’s input sensitivity pots have become noisy. One quick improvement technique that I’ve used is rapidly adjusting the pot from min to max — a zillion times. Leaky capacitors in the switcher can also be an issue, accelerating pot degradation and throwing off the switcher’s timing. I have some switchers that have been in service at least as long as yours and one of them needed a pot “wipe”. For me this “wipe” operation is instinctive preventative maintenance for older equipment. I’ll “wipe” every pot that I can reach.