Sounds silly - but - disconnect it - put it in the car, take it for a short drive around, then try it again.
I've seen a TV once with a similar steage gaussing-up problem - a woal around the house and putting it back in a slightly different place eventually cured it.
Much to amazed mutterings from the customer who had to help me carry it around.
Sometimes it can be something metallic (steel) - like a filing cabinet, which is not obviously magnetised.
Or it may even be an odd problem with the degaussing coil in the monitor itself.