Cpu overheating before and after changing thermal paste and cpu cooler

[Ackiii]

Hi,
I had a stock intel cooler on my i5 3550 and played a long time without problems one day i heared my fans very loud and touched the case and i was very hot.
So I stopped playing and shutdown my pc after he cooled and searched for a programm to see my temperature from the cpu.
Got core temp as a programm and it showed me that my pc is on 50 C° without anything open as soon as I start a programm it went up to 70 C° and when I played games up to 100C° so I stopped playing and searched for a new cooler.
I got me then the Arctic-Freezer-7-Pro-Rev--2 and cleaned the cpu and put the new cooler on again and started the pc. It showed me 35C°. When i start a programm like Chrome it went to 40-45 C° .
I was happy but as soon as i started playing a game i went again up to 75-80 C° and I'm asking myself why.
So if anyone has an idea why i would appreciate if he could answer me and Thanks already

 

[Aladdad]

Did you make sure you cleaned the thermal paste off correctly? make sure the cooler is seated properly. sometimes when thermal paste is not properly applied you might get an okay idle temps but under stress temps gets pretty high.