What's the most reliable CPU temp measurer that there is?

Core temp isn't working too well for me. It jumps by about 5 degrees Celsius every 2-3 seconds so I don't think it's very accurate.

EDIT: I started to mess around with prime 95 to see what would happen with the temps and they from bouncing between 35-45 to 80-90 in about 2 seconds flat when using coretemp. Could I have bad heatsink or does this seem legit? I'm using an i5 2400 overclocked to 3.6 gghz