3B1 green screen???

[John Obrien x4089]

I am a little curious about why my 3B1 doesn't boot up, but comes
on with a green screen when cold.  After I let it warm up then shut
off and back on again a few(sometimes many) times, it will boot up
successfully.  I know this is not a freak occurrance because I bought
another used one last year which does the same thing.  (I just leave
it on all the time).  I figure it must be the power supply or the
hard drive.  Is this true and what is the remedy?  Refirbushed disk??