iMac 5K Bootcamp Black Screen when playing
By the sounds of it your iMac is overhheating.

A temperature of 110° is too much so your graphics card stops processing for some time in order to prevent any lasting damage to it. As it is only your GPU restarting and throttling, your applications are still open.

As the iMac is an all-in-one, some airflow and heat capacity drawbacks have to be taken.

In order to prevent this from happening you could do the following:


decrease the graphics settings
open your iMac and clean the air intakes and outlets to improve ariflow


But remember. The iMac was not designed for gaming on windows, so your experience will never be perfect. Drivers that have been tinkered with might work but should not be considered reliable.