Sounds similar to what my Lenovo Yoga 13 (and thousands of other users) experienced.
I'm assuming you have an Intel graphics chip on that system. Assuming that, Intel has its OWN power saving settings along the lines of Adaptive Brightness/Contrast.
Take a look at: http://answers.microsoft.com/en-us/windows/forum/windows_8-desktop/how-do-you-completely-disable-adaptive-brightness/b0161873-4ebb-4070-bdad-fdcc01f17e16?auth=1
Also... Toshiba just announced a recall for certain Satellite, Tecra and other model systems manufactured between 2011-2016. Have a look for more information on that (which is not related to your issue, but a concern) here: https://batterycheck.toshiba.com/?region=TAIS&country=US&lang=en
Hope this helps somewhat.
