((Signal Error)) Need suggestions

This my my 2nd replacement of this motherboard ((ecs p4m800 pro 478)).
The first one when installed would give a "signal error" so I rma'd it. The second one would do the same thing but I was able to actually get a screen after I cleared cmos. I was able to install windows and it ran fine. That was a month ago. Now all of the sudden I am getting the same signal error message on the screen. Checked RAM/GPU/ - - using 350w psu/2.6ghz celeron 478. ECS tech support phone number is no longer valid so I am unable to contact them for help on this issue. I could not tell you why this happens. I try onboard video also and still same thing. I have tried all add on components on a different computer. I also test computer on different monitor and get same message.

Your suggestions would be most appreciated.