It's almost certainly a board-level problem with the analogue video circuitry. I'm far from the most knowledgeable here about these things, but from my Color Classic modding experience I'd say a failing (or failed) component on the main video board is resulting in insufficient voltage being sent to whatever controls the projection of the horizontal width of the display.

Others more expert than me will have to tell you whether this can be fixed, or if it requires total replacement of the analogue board.

Best,

Matt