It's not normal to be idling at those temps even with the stock HSF, I would hate to know what temps would get to under load. I would reseat the heatsink with new TIM and see if it helps. Make sure you scrape off the old TIM and clean both surfaces with rubbing alcohol. If a reseat doesn't help, then depending on how "quickly" the temperature fluxuates it very well could be a faulty thermal sensor or something.
Main: i5-3570K, ASRock Z77 Pro4-M, Asus GTX660 TOP, 120 GB Vertex 3 Max IOPS, 2 TB Samsung EcoGreen F4, 8GB 1600MHz G.Skill @1.25V, Silverstone PS07B
HTPC: A8-5600K, MSI FM2-A75IA-E53, 4TB Seagate SSHD, 8GB 1866MHz G.Skill, Hand-Built Wood Case