The temperature you mention is quite high, taking into account that many motherboards have a low warning setting of 65 degrees C.
Don't forget too, that the temperature you're reading can lag behind the ACTUAL temperature of the CPU core because of the thermal inertia of the HSF. This effect can easily mask sudden increases in temperature, but the fact that the temperature is so high to begin with suggests you're pushing it's limits!
I suggest you
# apt-get install memtest86
reboot, and run memtest for a few (hot) days. (Yes, I said DAYS!
Make sure you:
- have heatsink paste on your CPU HSF
- clean dust from all fans and surfaces
- tidy cables for better airflow
A properly designed case will keep cooler with
the panels on provided airflow is sufficient.
If you are unable to reduce the temperature significantly I suggest you replace your HSF and/or install an extra case fan.