Error: "PCIE High End Graphics card not connected"

I have a dell dimesion 8400 and from time to time  I get this error during power up and it won't boot up (PCIE High End Graphics card not connected). The error flashes red on the monitor and there is a beeping. I typically press the power button 3-4 times before it eventually boot  up, but it seems to be getting worse

I had the power supply and video card checked out  at a shop and everything looks good. I also updated the video card driver. It's the original video card shipped with the computer.

Does anyone know what is causing this error?

thanks

P