If its not the temperature that shuts it down - the temps you post seem uncritical - , then the PSU might be the bottleneck and lose its capacity [over time... 4 years ...]
Cheapest thing i would go for is the renewal of the cooling paste on both CPU and GPU. Maybe its the 550 that is causing the shutdown signal. [i doubt it... usually they give off signs of overstress like pixels and funky artifacts... but i have not had a lot of dying graphics cards on my table dso far]
Take a look at it - or find yourself with a dead machine sometime soon in the future. Not something you need right now...?!