iPhone freezes on lock screen!

well this is my 2nd iPhone
my phone will be on the locked screen and will go through the highlight of "slide to unlock" but then the second time it will freeze on the S

i have no idea why it does this. it happend on my firsy phone and i thought it was a defect...

so when it freezes, it stays like that for about 10 seconds then the screen shuts off then i do the same thing again but i have to slide my finger across before it freezes.. then it goes to home screen and stays frozen for about 10-12 seconds...

i have a 8gb iPhone and about 3 gigs full, so i have no idea what it is

any help guys? thanks