Update your post to include full system hardware specs and OS information.
Include PSU: make, model, wattage, age, condition (original to build, new, refurbished, used)? History of heavy use for gaming, video editing, or bit-mining
Disk drive(s): make, model, capacity, how full?
Use Task Manager and Resource Monitor to observe system performance. Use both tools but only one tool at a time.
Objective being to discover what happens or changes when performance drops and temperatures increase.
Has the case ever been cleaned to remove dust and debris?