Jaistanley ..... I'm experiencing the exact same thing. And ONLY with Logic Pro X!
For anyone who can help me out, here's what I'm using and what's happening:
Hardware: Mid 2011 iMac 2.5Ghz Core i5; 4GB RAM; OS X 10.9 Mavericks; 7200rpm internal HD.
Software: Logic Pro 9 and Logic Pro X.
Problem: Logic Pro X keeps stopping during playback, providing the "Disk is too slow or System Overload" message. However, I can open Logic Pro 9 and play, edit, etc... without any problems.
Both are configured to the same preferences and settings: 24-Bit recording; 48Khz audio; 256 buffer size. I've only got 5 audio instrument tracks playing about 24 bars of midi notes.
It stops like every 4 bars!!! ***???
I opened an older project of mine in Logic Pro 9 that has about 20 audio instruments, plus 12 tracks of recorded vocals. My current project has no recorded vocals at all, so it can't simply be my hard drive. I know it's good practice to record to external drives, but I've been doing this for years, and can still record to the disk in my old 2006 Black MacBook that has a 5400rpm HD using LP 9 without a glitch!
Any ideas? Is LP X just that much of a hog on resources that my paltry 4GB of ram isn't enough when it seems to be just fine for LP 9?
Any answers that aren't redundant would be of help.