@runonce: Thanks a lot for your valuable advice. I cleaned the machine quite regularly and hope that I did the job well. But you never know...
The aforementioned instability manifests in a all of a sudden freezing of the entire system. This may occur after one minute, X minutes even the next day after starting the PC. I observed that this issue happens after the use of Foobar or VLC media players (I already deinstalled them, installed new versions, used various backups of which I know that they are running smoothly etc.). Nothing solved the problem. As I do not want to waste a lot of time, I thought it might be a good chance to upgrade the entire system.
But maybe my issue sounds familiar to you or anybody else on this forum and you want me to know how I fix it, then please feel free to send me your advice(s).
Thanks again