For complex, lengthy, high-def projects the 64-bit version is best.
Allowing a project to fully cache normally prevents buffering. Let the green line above thumbnails complete. 
The band just below the preview window may change color, depending upon focus and project complexity. These actions can take time. Adding or changing transitions and effects may prompt brief buffering, as new parameters are cached.
Verify that the video driver and Direct-X are current... https://www.nch.com.au/kb/10265.html