The Vcore should be as stable as possible. I myself don't see any variations when going from idle to full load. The mainboard capacitators should prevent that from happening.
I don't know what the limits would be but I think instability could occur with a lot of fluctuations especially if it goes down (it should never be too low, that will make you crash constantly unless the cpu is good).
CPU speed is voltage dependent if you overclock. Sometimes the CPU needs a voltage boost to achieve a higher overclock. This also works the other way around, some people lower the voltage to make the CPU run cooler at a lower clockspeed.
I like stable voltages in operation and only change them in the BIOS. I don't like fluctuation in operation especially not 10% (unless of course the CPU supports it like mobile CPU's)
Most newer chips, especially the slower or mid-range ones, will be happy to run at stock speeds with slightly reduced voltage. It's a good thing to experiment with because it can have a dramatic effect on heat output.
Each individual CPU will need a certain amount of voltage to reach a certain clockspeed. This can vary by a huge amount (My T-bred 'B' will do 2Ghz @ 1.4V, but many other t-bred b chips need at least 1.6V to get there), so the manufacturers have to choose a default voltage that's likely to work for all the chips in each speed bin. This means it's quite likely that your CPU will do stock speed with less Voltage.
Of course if you overclock your CPU by much, you'll start needing increasing levels of Vcore to remaing stable at the higher clockspeeds, which means more heat output.
hmm.. think I mis-read that one. I was thinking you were referring to some undervolt/overvolt BIOS options...
Now I think it's talking about the disparity between the selected voltage and the actual voltage. It's extremely common for the two to differ by a little, (i.e. you select 1.4V, and it actually supplies 1.41V or something). 10% sounds quite high to me, but it just means you have to allow for it when manually adjusting the voltage.
Continuous <i>small</i> fluctuations in Vcore are also common, but these should definitely not be 10%. more like 1% or so.. otherwise you'll have an unstable system. A lot of overclockers have problems reaching higher clocks because the motherboard can't supply a stable enough voltage above a certain level.
"Sex without love is an empty experience...
But as empty experiences go, it's one of the best" - Woody Allen<P ID="edit"><FONT SIZE=-1><EM>Edited by ChipDeath on 02/07/05 09:42 AM.</EM></FONT></P>
The prescott chips change the voltage themselves. This is why a lot of old boards cant deal with them. I dont know if Intel built this in, as a way of keeping heat down, or if it's just because of the draw down that happens normally unde heavy load. It is a requirement of prescott boards.