Factors necessary for steady pass rate of UI testing of WPF application [on hold]

This question is rather a broad one and I would like to get ideas and opinions about this.I am testing a big WPF application that uses complex REST services and database calls. This application follows MVVM pattern to the core.

I have written a complete UI Test framework for this application in C#. I would like to believe that I have taken utmost care to follow patterns like PageObjects, Repository and also maintained the SOLID principles. But still when I run my test cases most of them fail for unsteady reasons.

My ask is, what would be the factors that I need to follow or keep in mind so that my test suite is a steady one and at least have pass rate of <80%?

Little bit more about the test framework (don't know if this info helps): Written in C#; Used CodedUI; UI has all sorts of WPF elements possible starting from TreeView, Grids, Document manager, cascading TreeViewItems and ofcource basic elemnts too like textbloxk or textboxes;test cases are written as Unit tests in VS; have created pageobjects for almost everything to rule out high dependency.

Please let me know if any more information is required. Thanks!