Hello,
sorry for my English 
A few months ago I connected an old HP w19 Wide LCD Monitor 1440 x 900 60Hz to a HP Compaq DC7800 (VGA cable) and
everything worked fine except for some occasional add/remove hardware sound (Windows 10) which, however, didn't affect the pc performance.
Now I'm having trouble with my monitor which flashes almost all the time. When it doesn't the monitor works, colors and resolution are ok.
When the computer is on both the monitor and the AC adapter leds flash, and the same applies when I turn off the pc (they keep on flashing).
If I turn off also the monitor the adapter led is always on (green) and it doesn't flash.
The VGA cable is ok on other monitors.
Any kind suggestion? I hope it's just a malfunction of the adapter.
Thank you!