I have an Hewlett Packard MX70 Monitor. When I power up this monitor the screen is flickering in an horizontal angle. If I give it a good bang on the side, the fickering stops temporarily. What could it be?

it sounds like to me it's either a bad solder joint, loose connection,
or just a failing tube.

the fact banging it fixes it indicates to me something is loose. with
close to a thousand solder joints in that thing, have fun.

i'd start out with the obvious things..check pins on connectors, cable
connections...anything that looks like it'd get some vibration.