The E8400 does not play nice with many core temperature programs. Use a programm called realtemp 2.4 -this program does report the accurate temperature. At first I thought I was roasting my E8400 till I found this out. Now my overclocked E8400 (3600ghz) runs at 40 idle and 64 under load.
remember to use thermal paste cleaner to clean off the old paste.if you havent got any alcohol cleaner you could also use vinegar to clean the base.but dont leave the vinegar on there for too long.rinse after vinegar clean.