It would do that if a process is demanding the CPU resources.
Open Task Manager, click on Processes tab, click on "CPU".
That will sort out the processes with the most CPU intensive process at the top. There you can see what is using 100% of your system. If its the game you are playing (Gamename.exe) then its all good as there are times when some games use full CPU power. If its some unknown process, copy its name and post it here. We can identify what it is, if its safe for your PC or not, and if not then how to remove it.
These steps should pretty much solve your issue.