Hi
there's just something really nice about old retro stuff isn't there, sadly collecting old stuff comes with the responsibility of maintaining these things.
just by going off what you have said so far I believe your issue may be a heat related issue, my advice would be to dismantle your console ,give all the fans and heatsinks a good cleaning out but last but not least give it some new thermal paste ,thermal paste usually helps with these sort of issues as it needs replaced every 5 years or so.
if that doesn't work try looking for any blown components or bulging capacitors as they can be quite common on consoles of this age
hopefully this helps!
Thanks:-)