Do you have Photo Mechanic's Preview preference page set to use the Best ( uses more memory ) option in the High DPI Display Quality menu? By default, this menu is set to the Standard (fastest) option. Check the Caching preference page to make sure the Memory Cache size is 400 MB, not the default 256 MB. If the Preview window is still displaying re-draw issues, try the R shortcut key to Restore the window's default settings.
--Bob