My Cpu Temeratures is kinda trolling going from 90 degree to 60 when I

So, I have a gaming pc that I built 4 years ago, i7 4790, MSI gtx 970, 8 Gb RAM ,and I tried to kept it as clean as possible, I was playing with it all the time without any problem and it never get ovet 70 to 75 degree celcius at heavy load , until last week, I was playing the game blair witch, and it is not a super demanding game, and I noticied that the cpu is at 100 degree on 4 cores, so I tried to fix this essue by replacing the thermal paste and re-seating the heatsink properly, but sadly, nothing has changed, my cpu reached 100 degree is just 2 minutes and kept working without any noticeable problem and games still smooth, no lag , no fps drops nothing . I almost did everything possible in my hands , cleaned my case, reapplying the thermal past 3 times, arranged my cables to give more space, but nothing was helpful, but found out that when I remove the side of my case the heat is decreased dramatically from 90s to like 50s or 60s at most, at stayed there for more than 3 hours of gaming and I tried to play some heavy games like gears 5 and assassin's creed odyssey, it never get above 69 degree. but the moment I put the side of the case back, it start worming up quickly and reached 90 degree, that's so weird and I don't know what is happening exactly. there is two things I did before this start happening, I first changed my PSU from 450w to 550w, and I updated my bios to the latest available wich is from 2016 for my motherbord.

please if someone has the smallest clue of what what is going on in my rig, please help I'm totally out of ideas :/