Why does the screen on my Dell C600 laptop only work sometimes?  It
works fine with an external monitor, and when it works, it works
great.  Other times, it does not work.  There is a slight ghost of an
image of what it is on the screen when it does not work.  I have
tightened all screws and cables.

I have done some searching that recommended updating the BIOS - but my
BIOS is already up to date (A23).  Another thought related to the
video cable, but this would not seem to answer the completely random
times that it does work perfectly.  I also do not think it is the
video card (1) because of the fact that it works sometimes, and (2)
because an external monitor always works.  I also do not think it is
the screen itself for the same reason.

I am technically knowledgeable and able to follow direction well. 
Please provide detail in the answer.

Could you clarify a little -- though I shall leave (for a day) for a
researcher to pick up the dosh. Does it lose the backlight?

Best