SP-303 sometimes doesn't turn on, peak light + Err screen

[mistermanager]

This has been happening on and off for a few years now but sometimes my 303 won't turn on, and it seemed fixed for a while but it keeps happening again. It used to turn off while I was using it too, but it doesn't seem to be doing that anymore (can't tell for sure though since I never use it long enough for that to happen anyway)

I noticed that the peak light will sometimes turn on, and I would sometimes get the "Err" error screen, so I tried reheating the solder on the power input and power switch, which kind of fixed it for a while (it'd turn on every single time, now it's back to being iffy), and when I looked at the board neither of those seemed to be loose at all either. Lately I've been getting the peak light and error screen again, but at least it's staying connected while that happens? Not sure if that's a bad sign or not

Has anyone else had this problem? I'm kind of hoping to fix it so I can use it more again, or maybe sell it since I haven't used it in so long (still wanna at least know what's wrong with it before I sell it, even if it's not something I can fix myself). I'm hoping I just need to re-solder those two parts again, maybe I didn't heat it enough when I tried that? I'm guessing it's related to one of those since that did seem to fix it for a while, but it started happening again. I did read that the power input is often the culprit in an older thread on here, but I still don't know how common it actually is.

Thanks for any help!!