shadow display on flatscreen

This morning, after turning on the PC and flatscreen, I noticed that
the display showed a shadow picture: as if the whole Windows desktop,
with icons and all, was show twice: the normal display and one moved
DOWN an inch or so.
The shadow was faint, not at all as clear as the normal display.
I turn the screen of and on again, and the effect was gone.

Special circumstance: High room tenperatures for the last days: 30/86
C/F, just maybe 35/95 C/F.
No problems reported by PC. That one seems to stay cool enoug... dunno
about graphics card.

Anyone seen it before ?

(It's a Dell screen)