NEC AccuSync 120, 21" inch monitor, pre warmup vertical stabliaztion issue
 

I have an NEC AccuSync 120 when I turn it on in the morning the
display will be about 1/4" taller on the top and bottom than I have it
set. Suddenly the display will zoom in, expand out at the top and
bottom, just as it gets most exagarated the CRT goes dark but the
green power light stays lit. After 5 or 10 minutes when it is warmed
up, I push the power button and it works normally and the image is no
longer 1/4" larger at the top and bottom.

What could be causing this?