3 monitors do the same thing on the 1 desk top i have.

Hello, im David.

I have a dell desk top that is about 4 to 5 years old and has always worked great. i do shut it down when im not using it. in the past few months i have been noticing that my monitor will trun off by its self every couple of seconds. At first i thought that it was the computer monitor it self. so i tryed everal different montors all known to be in good working order. they all did the same thing. I believe to assume that the issue is with the desk top its self. its a Dell desk top model number DX4860 running Windows7. i dont know what to do to fix the issue. Someone please help me.