My Blackberry 8310 won't stay powered on....?

[Douglas]

I bought a blackberry 8310 from a guy I found in the street he said he didn't wanted it no more he said he wanted 10 bucks and i bought it but I came to my house and put it to charged and it charged but when I'm using it don't matter what I doing it would power off and would not turn back on I tried a new battery but it would still power off I even bought a new memory card for it but no use and try to get a new OS but still it won't stay on............... any help? plz