Was this after a recent update? If so, go back to an earlier version. It could just be a glitch with a nightly if you're doing that. If you haven't been updating recently then.....
Check the kernel governor. See if it's set to peformance or lagfree or something like that.
If that isn't it, check battery stats to see what is consuming the most power. Use a 3rd party app like Better Battery Stats instead of the one that comes with Android. See if something is always on. You could have a wakelock that's killing you. If you do, ditch that app and see if it improves.
If that doesn't help, reinstall the rom. Dirty flash over the one you have. See if that fixes it.
If that fails, do a clean install of the rom and add your apps back 1 by 1 until the problem is triggered. Then you'l lknow what's causing it.