my medion monitor goes black with computer still running?

My computer has a Ati radion video card:its 3years old:it just started happening,as far as i remember,the screen seems to shut down:cable disconnect symbol comes up and screen goes black:computer and any programs running,keep running:I hit standby button and i hear programs and computer stop:I hit button again and screen comes out of stand(and computer)but screen is messed up ie desktop is like gobbledegook:it stays like that for v.short time and goes black again.
So,I go into safe mode:control panel:display and run thru' most:then back to mode selection and go into vga mode,control panel:display:and run thru' most settings;I restart and go back to normal mode;computer runs fine and then goes to black screen..........