Is it a CRT monitor? Or the old style screens?
See when I switch my monitor on, it farts
'de-gausses' then I get a wobbly picture. Then a second later I hear a relay 'click' and the wobbling goes.
The last time I had a wobbly picture I waited for that relay to click but never happened. I checked the monitor plug that plugs into the graphics card and saw that a pin in the plug was bent.
Once the pin was straight the monitor worked.