Successful slappage tells you that it's most likely a loose connection. ... In your case, pay special attention to the CRT socket itself (not the HV "suction cup" connection -- I mean the socket at the rear of the CRT). A little investigation, and you should be able to get that one to behave.

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I agree this is a likely cause of problems. Often the glue/gook used to keep the video board secured to the CRT neck socket dries out (or has been cut by someone servicing the unit in the past), making the connection liable to work loose if the machine is transported or jostled (or if it just sits in a place with temperature and humidity variation). A little hot glue, applied while gently pressing the video board onto the CRT socket to make sure it's fully seated, might do the trick. (All of this happening with the machine turned OFF of course!)

M