Please let us know if you are using a task killer. Or some type of system panel, OS monitor, etc.
See, to those applications, a CLOCK app is running way too long, and those task killers kill anything that has been running for a certain amount of time. It may think the clock has went into hibernate mode, and killed the application.
I have an analog clock on my main page, and every time the phone goes to sleep mode (screen off), the clock stops running, but when i wake the phone up, i actually see the clock hands (hour and minutes) move to syncronize with the time, so that means the clock goes into sleep mode when the screen shuts off, and then it wakes back up and syncs time when you wake it up. It does that to save battery, and also does that because that is the way the OS built it. So if you have a killer running, it's going to kill the clock after a certain amount of time inactive (say 1 hour), and then all your alarms will not run because the clock has been shut down.
So i explained a lot, and talked a lot here, but the short question is the same from the other poster, do you have a task/app killer ?
And same sezaro said, if you are using ATK, or other task killers, please make sure to exclude the clock from the list. ATK is good at learning what to close and what not to close.