Software Problem:
1. Soft Reset, hold power/lock button and toggle the ringer switch 4 times.
OR
Hold Orange Button + Sym + R
If that doesn't work and the problem persists...
2. Do a hard reset.
Go to "device info" app on the third page and go to more reset options.
Do a partial erase first. *Make sure you back up your info first*
If that doesnt work, do a full erase.
3. Contact Sprint or Palm, ask them about your problem.
4. Go get your pre replaced.
Hardware Problem:
If phone resets when closing or opening slider, when you set it down too hard, or if your phone appears to turn off randomly while in your pocket:
http://www.palmpreforum.org/palm-pre-news/slider-crashes-inspire-macgyver-solution/Otherwise:
1. Take your pre to a sprint store and get it replaced.