|To test it from cold, disconnect the monitor from the PC and then turn it on.|
You should get an on-screen-display saying something like Signal out of range.
You can access the monitor image controls. Is the image distorted? I suspect it will be. CRT monitors have controls to shape & focus the Cathode Rays ( Electron stream) and I suspect your monitor is having problems that are evident when cold but correct once the thing warms up.
Repair is not economically viable. Time to invest in a new monitor
Goin' Fishin' (Some day)