Such a weird glitch, but BIG impact on editing ☹ Any insight??

[Winston A. cely]

Have you tried the following (?):
Export out an XML of only your edited sequence, not your entire library.
Close the current library.
Quit FCPX.
Shutdown and restart your computer.
Open FCPX.
Import your footage into a brand new Library.
See if it plays correctly, and then if it does, import your XML. Relinking to all your footage.

Winston A. Cely
Editor/Owner | Della St. Media, LLC

17″ MacBook Pro | 2.3 GHz Intel Core i7
4 GB RAM | Final Cut Studio 3 | FCPX | Motion 5 | Compressor 4

“If you can talk brilliantly enough about a subject, you can create the consoling illusion it has been mastered.” – Stanley Kubrick