generally if you tap on the side of the cab and the monitor does anything other than normal operation, you're looking at a loose component somewhere. it's either something that plugs in, or more commonly, a bad solder joint.
when you're dealing with older monitors, the solder joints tend to crack, and that's what causes intermittent connections which will result in the monitor outright shutting off, or maybe losing primary colors (red, green, blue) or in other instances it could lose sync too.
the only way to know for sure is to remove the monitor chassis, neckboard, and if applicable, the remote adjustment board, and just look at all the solder joints and any that look suspect, you can just re-melt them. I've had times where the solder was very dirty and shitty though that I just use my solder sucker to remove the old solder and use new solder.
from my experiences with what I THOUGHT was cold solder, I probably have a 90% success rate with just reflowing solder joints. one interesting one was a K7000 of mine where the neckboard was partially cracked and the resistor in the heater circuit just kind of came undone. the tube would just shut off cause that circuit wasn't working, thus no juice going to the tube. shotgunning the entire neckboard with new solder corrected that.... until I blew that chassis up (and fixed it again).
as always don't hesitate to ask for help.