Stock fans that are sold with CPUs (notably those from Intel) have a very poor mounting system, using plastic "easy screwing devices" that frequently do not support elevation of temperature (it's then frequent to open the case and see that these pseudo screws are in fact no longer attached! sometimes that plactic has also cracked over time because of temperature and the fixation is in fact completely ineffective). In that case, the thermal paste on the CPU is not even in contact with the support of the fan and when it gets hotter, it is evaporating all around! heat dissipation is then very poor.
Consider using a better cooling fan with a more stable mounting system, with metallic screws (and springs around screws, plus some carton roundels between the mobo and the fixation to avoid damaging the mobo by excessive pressure: the springs around the screw will offer just the required elasticity to adapt the pressure to the normal dilatation of metals due to heat, and will also offer protection against vibrations caused by the rotating fan).