trouble w/ iphone locking and shutting down

[craveone]

hi there, i am not finding the answer i am needing here...a friend of mine gave me his old iphone, i have the phone set to never sleep or lock...when i manually lock the phone it will shut the phone down after a while...like locking the phone causes a problem...anyone else get this or know of a way to get around this...i end up leaving the phone in my pocket w/out the keys being locked, and it kills the battery cause the screen never dims or goes blank...help please!!!