Question Lenovo g500 strange boot issues

Jan 10, 2021
Hi there, I'm New here.
I have been experiencing an increasingly serious bootup problem with my Lenovo G500 laptop. Both the hard disk and the DVD drive are not showing up in BIOS, so it appears that there aren't any bootable devices. But other times they appear out of thin air, and the computer just boots up and Windows starts. Unfortunately, this happens only after 1-2 hours of switching it on and off, and this tiresome ritual is getting longer every time.
Notebook: Lenovo G500 (i7-3520m, 16 GB ddr3 ram, Samsung 500 GB SSD 750 EVO) Bought in early 2014.
Condition: Thermal paste was applied about 1 year ago. There are no heat issues.

What I have tried so far: I tested the computer with different hard disks, but nothing changed. I tried to use another Lenovo power adapter, but nothing changed. I run the Windows memory diagnostic (when successfully booted). I tried inserting a disc into the DVD drive, it spins, but it didn't appear in Windows. USB boot doesn't work, but it never worked with third motebook.
What I haven't tried so far: Lenovo System Recovery, but I want to leave this option as a last resort.

My questions are: What could be the root cause of my problem? What can I do? What else to try?
Any guidance and help would be appreciated.
Thanks in advance for your answers!