If you had three with the same problem, then it might not be something wrong with the phone but something wrong with what youre doing, did you root your phone? Get rid of any bloatware? Do you use a task killer?
Yeah, but i asked if you use a task killer? Do you have any apps blacklisted in the built in task manager? It could be the task manager thats causing your phone to act like that, or it could be another app thats messing with your system.