years?
time to clean it(internally), also check the values of the psu(some bios say if a voltage is too low), could be failing and creating excess of heat on some parts
does that case has a fan in the back, under the psu?
system covered and in a not proper place will get to 70° c easily, a proper place for most cases is top of the table, desktop you have your monitor, if you put a case too close to the wall, the heat will not be dissipated and system will reabsorb the hot air increasing temperature faster, a system with that cpu should run maximum at 50° c
also, what kernel are you using, video card you have and driver used?