The CPU temperature reading in BIOS is about 10~20 degrees centigrade hotter than the reading in OS
That is normal as BIOS does not send idle command to the CPU, making most of the power saving features useless. You should be getting similar reading if you disable EIST/C1E/CPU C3 Report/CPU C6 Report in BIOS.
As Intel specification clearly states Vcc (CPU Vcore) should drop in propotonal to the increase of Icc (CPU current consumption), in order to prevent causing permanent damage to your CPU. When CPU is under stress (in other words, under higher load), the current consumption of the CPU will go up, and the CPU Vcore will then drop to conpensate this change. This is a perfectly normal behaviour, and will not effect the overall stability of your system.
For more information, please refer to Intel CPU datashee, under "DC Voltage and Current Specification".