Nokia monitor's size and shape keep popping back...

Hi. I have a 21" Nokia 445ZA CRT monitor. Granted, it's probably about 6 or 7 years old no, but it's still working well. But for some reason, about 3 weeks ago, the vertical size/shape just suddenly pops to its default setting (I guess), leaving a black stripe down both sides of the screen... Like it all of a sudden could not hold the settings for "width", and decides to "pop" back to its default settings, requiring me to reach for the adjustment button in order to "stretch" the screen back out so as to lose the said black areas on both sides.

Is this just age - and should I reach for the pocketbook (which is slim right now...), or is this fixable ?

Thanks very much