Mackie HR624 come on and off mute for several minutes, and then just stay on mute

I bought a pair of Mackie HR624s ~15 years ago. So, these are some older monitors, but I only got to use them for a few years, before they stopped working. One day, one of them just suddenly went on mute, and the LED switched off. Then ~1 second later, it came right back on. Then a few seconds later, it would cycle on/off like that again. Eventually, it started happening to both of them, and after a few weeks, they began to switch on/off about once per second, and then just stayed on mute.

Notice I say "mute" instead of saying they’d turn themselves on/off. The reason for that is, these monitors have an on/off switch on the back, and they have a mute button on the front with a green LED that switches off when the monitor is on Mute. Since I’m not an electrical engineer, I have no way to know if this is a "mute" problem, or a "powering on/off" issue. I eventually just gave up hope and put the monitors in a box, and put them in a closet for 10+ years.

Now, I am at a place where I really need a descent pair of monitors, and I’d love to be able to use these and not just throw them away. I pulled them out recently, and when I turned them on… they seemed to be working fine! But… an hour or so later… 🙁 same shitty problem. Switching on and off, and then just died again.

I think these monitors have a power saving feature where they go on mute after X-min. of not no signal. I cannot help but think that these monitors are just fine, but there is some kind of weird glitch happening that can be fixed. Have any of you guys heard of something like this?