Hi,
What do you mean by 6pins and 8pins?
What is your power supply and where have you got it connected?
If it drops to 3.9V then I would say your 5v line is over loaded.
If your supply is providing current to the DC socket, then you are possibly overloading the onboard 5V LINEAR regulator.
If you supply is via the DC socket, then it should be 7V or higher to give the LINEAR regulator some headroom to provide a stable 5V.
The schematic you refer to, is not your schematic, PLEASE reverse engineer your project and post the schematic.
Where are the ON/OFF buttons and how are they wired?
Does ezButton turn on the internal pullup resistors if your buttons are from digital input to gnd?
If your buttons are digital input to 5V, then you will need 10K pull down resistors between each digital input and gnd.
Tom... 
PS. Off to bed very late here.. 