Have you tried power-down, disconnect monitor from computer, reconnect to computer and restart?
It sounds like a loose connection on your monitor cable.
Try this... with the computer on as it is, GENTLY wiggle the cable very slightly (just a little pressure side to side) and see if it affects the screen. If it does, it’s probably the connection. If it does not, try the cable connection to the monitor. If neither connection is loose, it’s probably the monitor itself.
I tightened the cable on the monitor. No change. Should I do the same on the connection to the computer?