i7 CPU Thermometer breaking down?

I have a custom build with a Intel Core i7-3820 on MSI X79A-GD45(8D) X79 motherboard.

I'm using Thermaltake CLW0215 Water 2.0 Performer as CPU cooling.

I have this build for about a year now. Back then, when starting my PC, the CPU temperature at idle would be around 27-30C and would never go over 60-65C under heavy load.

However, this week when starting my PC, it showed 55-60C on startup and could peak at 100C under heavy load. It looks like it's totally unable to go under 55C now, even if nothing is running at all. Using Power 95, I noticed the temperature rise very quickly, but also drop sharply when the test stops. Does that mean the cooling still works?

Dusted it off all. All fans spinning... Can't be really sure if the water pump is working, as it never made any sounds to begin with. But I would guess if the pump never worked, I wouldn't have stayed at 30C... I would be at 100+.

Does the CPU thermometer can be stuck at +30C than normal? Is it even possible for a CPU thermometer to be messed up? What should I be doing to see if there's an error or if there's a material issue?

Assuming that the cooling system itself (fans, pumps, etc.) is running normally, this sounds to me like insufficient heat transfer from the actual CPU die to the surrounding air.

The most common way that could happen is if the thermal paste evaporates for any reason, and that can happen if the CPU temperature gets much too high at some point. It could also have been displaced, if the system has been exposed to physical stress.

I would apply new thermal paste first, and see if that helps. It's cheap, and it's likely to solve the problem.

Use RealTemp. Only reliable way to get readings (for Core series).
About the cooler... well if you don't overheat during a load, it sure works.
Even on idle the temps would go through the roof in minutes.

But you can:
1) Clean off dust from cooler. If you have a can of air, hold the fan so it does not spins up as you blow air on it. (That could kill the fan.)
2) If you have any PC shop around, or an experienced friend, get the thermal paste changed.

The sudden drop-raise is not a failure, it's just that's how it works.
Once you start something demanding, sensors will kick up ~ +10-15C to the temp.
When the load goes off, they drop by that much again. RealTemp won't do this though. It always shows the real temperature in cores.

Oh, and never forget about ambient temp.