Monitor Turning On and Off

Hi,

I have a new computer that I just built a month ago and recently my monitor will just turn off by itself and turn back on by itself. I checked all the cables and wiring and replugged everything and change Windows power settings to never turn off my monitor. None of this seems to work. I'm running dual Dell mons, one is 24inch(over 3 years old) and one is 19inchs (5 yrs old). This is happening to my main monitor the 24incher. What the weird thing is, I need to use the other buttons on the monitor to turn it on and off when I want close the computer but the power button doesn't do it. Like if I press the screen adjustments and brightness buttons on my monitor it shuts my monitor off. I just wanna double check my monitor has lived it's course before I buy a new one. Thank you for any help.