A Droop Mod Sounds Like It's In Order
If you are talking about the Asus board in your sig it most definitely is drooping.
As was stated, do not rely on software or BIOS monitors. They are completely inaccurate, and vary wildly from board to board, and vary even on boards of the same make and model. Measure with a multi-meter and see what it actually is at idle, then see what it does at load. I will gurantee you it is drooping. All boards do this, but Asus boards do it badly.
There are droop mods you can do to eliminate it. By drooping one has to raise the vCore higher than is needed to allow for the droop. If you droop mod a board, you can run at lower voltages because the droop is controlled under load.
The reason all boards do this is becasue Intel has a spec that most board manufactures follow. It is what I call "built in n00b protection" to protect the processor. It's a real pain for OC'ers that have studied and need the power! When a high load at high tempertures is placed on the CPU it will droop thereby allowing the processor to cool, but most experienced OC'ers aren't worried about cooling as they already have it, and monitor their temps. There is a catch to this "n00b protection"...
The catch is that when we OC, we have to keep raising the vCore to allow for the droop, and to keep the system stable, and this causes the CPU and the PWM section to actually run hotter at idle.
Intel mandates this spec to keep the n00b from cooking the CPU.
From what I have read, and understand the Asus boards are resistor based to comply with this spec. Basically to droop mod it, you have to bypass that resistor which is basically a "voltage control" and build into the board a variable resistor or "trimmer" that allows you to control this droop.
There are threads on doing these mods, and there are propbably speciffic threads for that motherboard that show how to properly do the mod on that speciffic board.
If you are looking to overclock from high to extreme levels, a droop mod is a must on the Asus boards. I have read many threads discussing this concerning the Abit and Asus boards, and they all say the same things concerning droop.
You get to a point of diminishing returns, because the droop is soooo bad, that for every increase you have to go up the droop just gets worse and worse once you get past a certain level.
On my Abit board for example, I can go to 3.9Ghz at a respectable voltage (1.4875), but to get to 4.0Ghz I have to go up another notch just for that extra 5MHz FSB, and yet another to get to 4.1GHz. At 4.2 the droop is so bad that even 1.5325 is not enough, and i won't dare go higher. 3.9 is the sweet spot for my system right now until I mod the board because the heat goes through the roof at 1.5125. At 1.5125 I only get 1.46v actual at idle and it drops badly under load. To get it to even POST at 4.2 I would have to raise it again, and that is not gonna happen in this lifetime...not for a 4.2GHz OC. Maybe for a 4.4GHz OC on Vapo, but that isn't happening on my air cooled rig as it sets now. I love my DO Prescott too much!
If you get a multi-meter and clock up in steps, and check at idle and load at every step, you will see the sweet spot, and the spot of diminishing returns.
Getting rid of droop will not only help to achieve higher OC's, but it will also help keep the CPU cooler by not having to use an inflated voltage to allow for droop. it will also help to lower PWM temps.
Sorry for book I just wrote, but I thought it may help you and others who are seeing this issue at the higher OC's.