I had this happen once, after I just got the phone. I think some part of Sense crashes and then the touch screen can't be used.
I don't think it's a problem with an app.
You should probably go through the bloatware on the phone and disable what you don't need, just so the processor isn't dealing with those apps.
Also you should not run a task killer even if your carrier installed one. (There ate a lot of idiots that work at cell stores that arbitrarily install these without even asking).
Mine only occurred once. At the time I blamed it on bluetooth earpiece, but I am not sure that had anything to do with it.
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