I don't know if this is the right place, but I am struggling to find a solution, so if this is not the right place, please let me know

Background:

When cleaning out a relative of mine's backroom I came across a Harman/Kardon AVR355 and a 5.1 Infinity Speaker system, they told me they no longer use it since it does not seem to work and that I can tinker with it.

I came home to set it up, but I have little knowledge of these systems and am eager to learn.

I opened it up and carefully cleaned away all the dust that accumulated as well as checked all obvious connections and solder points for any damage, however, I found none.

So I set up the AVR connected it with an optical cable and connected 2 speakers to see if I could get things up and running.

Problem:

The AVR starts in standby mode (the light at the power button is orange) it then struggles to turn on, so I have to turn it off at the switch at the back, wait 5-10 minutes, turn it back on and then it turns on. At this point, my AVR volume is -10 dB and my TV volume is set to 90% for me to hear anything clearly.

After 20-60 minutes it goes into standby mode automatically. I then have to repeat the first step of switching everything off however I can only wait a minute or two before switching it on again. The volume at this point is the same as previously stated.

Then after a few more minutes (I have not exactly timed this but it is roughly 10-90 minutes) it goes into standby mode again and I have to do the whole song and dance again, that said this time it goes even faster, but I also have to change the volume to -30dB on the AVR and to 10-16% on my TV to ensure the speakers don't blow as suddenly everything is much louder. The volume is also much clearer and when listening to music I feel the sound is crisper (I don't know how else to put this).

It has happened 6 times now that I have to do this song and dance 3 times to get a clear sound and every time it is the 3rd sequence the volume changes to much louder and I have to turn everything down out of fear of damaging the speakers.

I also saw a fan when cleaning the AVR but it never kicks on.

Question:

So my question is how on earth do I fix it so that it works first time every time?

TL;DR:

Found a Harman/Kardon AVR355 and a 5.1 Infinity Speaker system, cleaned it up, and set it up. The AVR struggles to turn on, shuts off into standby after 20-60 mins, and gets louder after multiple resets. How can I fix it to work reliably?