Size of CRT screen keeps changing - loose connection?

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Hi guys.  I need some specilist help here.  

I am in the UK.  The size of the visible part of the screen on my
17inch CRT monitor on my desktop PC keeps changing slightly.  

The screen jumpiness seems to come in bouts.  This suggests to me
that it may be due to a loose connection.  But shakes, tugs, taps
and thumps do nothing to help during a bout of unsteadiness.  Is
it reasonable to conclude it is a loose connection ???

Or could this be a sign that something more complicated is going
wrong ???  Maybe something which means I have to get a new monitor

Is this fault more likely to be in the CRT unit, the graphics
card, or the cable & connector ???

See below for background data.


===========  BACKGROUND DATA ==========================

Old-ish PC running XP Pro + SP2.

PSU in PC seems ok and fires up HDDs with no trouble.
Power is fed to the CRT by its own mains cable.

CRT is reported to be Tatung Mitsubishi V70 model VM71RDA.
Video card is an nVidia GeForce2 MX/MX400.

CRT colours and crispness seem unaffected.


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