Is it my CPU cooler installation or Tcase sensor

I just bought a new system, and had a IT guy build it for me. I'm a total newbie at building and OCing, so after reading the sticky's about quad temperatures and OCing and running the mentioned applications I saw something that made me wonder about the heatsink installation.

System:
Quad 6600
Asus P5Q-E
Cooler master Hyper 212
Ati HD 4850
Kingston Hyper 4 GB kit (800 Mhz)

After starting the computer my Tcase has a temp of 7 C (speedfan and others) and my cores around 19 C (Realtemp, Speedfan shows 5C higher).
(ambient temp around 15 C, cold nights , poor insulation)
After 10 minutes idle my TCase halts around 13 C and my cores around 25 C (Realtemp).
After 12 hours idle Tcase at 11 C and cores around 25 C, ambient at 20C, after hour video playback Tcase 17C.

After stressing (ambient around 22 C by now) for 10 minutes with FFT small, cores are steady around 55-60 (Realtemp) and Tcase around 40. After 10 minutes idle my cores go back to 29 and Tcase to 17C and not 5C lower as should be. This is consistent with several stress tests.

During installation the guy put a thin translucent film of thermal paste by rubbing the tube on both processor and cooler, after which he spread it further with his fingers. Before installation he wiped is finger on the processor, so a think gray spot of thermal paste was left in the middle.

Question:

Is OCing with these temperatures a problem or shouldn't I worry much about Tcase (callibration issue, temps below ambient, sensor stuck) and rely on core temperature with realtemp and substract 5C for Tcase or should I reinstall the cooler and redo the thermal paste (anyway)?
Tnx.


Edit: some rephrasing and grammar

The netherlands.. it's not that cold at night, about -5 to -10C, but it's a small appartment