I have the exact same problem with a Tecra laptop. We didn't want to spend money on it so just use it with another screen. Try these threads though, I tried most of these solutions and put ours down th the invertor. Funnily enough we have had this issue with three Toshibas!:(
Display problem with Toshiba Satellite Pro 4600 - Page 8
Display problem with Toshiba Satellite Pro 4600 - Page 11
it works fine on an external monitor!! ive not got an iverter that exactly the same, its from a toshiba with the same connections!but very slightly bigger, but it does the same
so i dont really know what it could be?
may be ambient light sensor:)