If you disable Intel speed step then you will have stable voltage!
Not quite. He needs to disable the C states. Speed Step lowers the voltage a little bit but not that much. Mainly it lowers the multiplier to x16.
Go to the BIOS and look for C1, C3 and C1E, disable them and it will stop. But if its stable with them on, no BSoD and such, leave them on. They just save power.