i havent read all the other answers but have you tried to call the following function?

https://msdn.microsoft.com/en-us/library/windows/desktop/ms686234(v=vs.85).aspx

or see the article of Bernard Bout
http://www.foxite.com/faq/default.aspx?id=55


this helped me alot! i call it in a custom carbage collectior witch also releases some not used objects i stored in a _SCREEN-Collection.