Apple Mac 5260/100 - Manual (Page 47)

Troubleshooting First Checklist - 2
First Checklist
Jitter, faint lines, or screen movement can be caused by external
interference such as electronic devices and fluorescent lights.
Move the unit to another room or building to help determine if
external interference is the source of the problem.
A misadjusted screen can mimic the same symptoms as analog
board or CRT failures. By performing the adjustment procedures,
you might determine if one or more of the adjustments is the cause
of the problem.