CPU temp question. Rig freezing up.

Need some help from way smarter people than me. Recently I’ve been having issues with my PC where it just freezes and requires a hard reboot. I was monitoring my cpu temp through iCue while gaming and I was 90 degrees plus Celsius. As soon as I exited the game it dropped 40 degrees Celsius. Is that even possible? My aio is 4 years old and it is a CORSAIR H150I ELITE LCD CPU 360MM. Could this just be a dirty radiator or needing thermal paste re applied or is my aio failing? Do I have another component failing as well? Any and all help is appreciated.