Title says it all.
OS X 10.9.5, relatively new install, 2 weeks old
AE CC 188.8.131.52
Late 2013 MacPro
CMD+Shift+"C" no longer pre-composes until I reboot the system, will work for a period of time, and then stop.
Kind of annoying, since I can't remember the last time I actually used the mouse to pre-compose something before today.
Check your system shortcuts or any utilities that may run in your menu bar. Something is usurping/ intercepting the keyboard commands.