On a regular Lisa, I'd remove the top, and there are 4-5 pots along the top of the analog circuit board that adjust horizontal and vertical hold, screen height and width, etc.

These pots can become intermittent connections over time. You can use some contact cleaner and rotate them back/forth while the machine is running to adjust the screen and clean up the connections.

Usual warnings about being around live CRTs apply.

If this is your machine with the Mac motherboard, this advice may not apply, since I don't know if you're using that part of the Lisa video circuitry.