Some Superset 420 phones were known to do this over a long time of use (powered up), the LCD panel just gets, for lack of a better term, "weak," and fades out.
Honestly the only thing you can do is replace the sets, preferably with 4000 series phones as these seem to stand up better over time compared to the 400 series phones.
Pretty sure the SS 400 series was discontinued well over 10 years ago, they have surpassed their normal usable life span, time for some new ones I think.