Refer to older post - http://forums.thinkbroadband.com/hardware/f/4388473-...
I would suspect airflow / poor CPU fan first.
Also bear in mind that thermal paste can degrade over time, but I wouldn't suspect that unless all other avenues have been explored. While 70c is quite hot, it's not worryingly hot. The throttle temp will be higher than that (guessing 90c -ish). That said, there's no harm in removing the HSF, cleaning it, and applying new paste in the recommended manner (small pea method).
Maybe instead of just talking temps, also refer to 'x' temp above ambient room temp, both idle and full load. If you're running it in a 25c or greater room, the idle temps will be high to start with, and CPU temps will rise fast. My room is generally 20-23c, but the central heating and the PC together can get the room temp up to 27c if I forget to turn off the radiator. The CPU's idle is generally 5-10c above ambient (two intakes, two exhausts, plenty of airflow).
Things to check:
1a. Airflow - Do you have equal amount do intake and exhaust fans, and is the exhaust air being vented from the top half of the case?
1b. PSU - Is your PSU venting into the case and adding to the heat, especially as older PSUs tend to get hotter with time.
1c. GPU - Is your GPU a 'hot' GPU, and does it vent into the case?
2a. Does the HSF feel firm, and tight on the CPU? You don't have to give it much wiggle to find out.
2b. Does the CPU fan turn fast enough to create airflow? Some fans turn, but airflow is minimal. If you have the option, increase the fan speed. Does the fan turn at all?
2c. If your BIOS has the option, set the CPU alarm to a temp that is halfway between full load expected (e.g. 65c) and throttle temp. If 95c is the throttle temp, I would set the alarm to 80c. It's a handy way to find out if the CPU is hitting high temps when you're not looking. Be sure to swot up on the load and throttle temps though. If unsure, err on the side of caution, and drop the alarm by 5-10c.
As I said in the linked thread, the E8400s have a history of questionable temps, but 64c on idle is very 'interesting', and if it's still doing that with the case open (if a tower, lie it on its side and let it vent up), there's something amiss with the HSF/paste or the sensor.