IPHONE Sleep mode Problems *help*

Hey guys , im looking to buy an iphone off my friend, now its been unlock with anysim i believe, and he's having problems with it.

Basically if you keep it in sleep mode for more than 2mins+ and when you get out of sleep mode... it will automatically restart... and if you just don't use the sleep mode. anyone else run into this problem?

there is a simpel fix for it? do u know how ? well the thing is i dont have much options, cuz im in canada, and its not even out here yet . onlly other thing is to go down to buafflo getting a brand new one 1.1.2. and using the stealth sim, that way i dont mess around with the software. what do u think?