Problems with sony ericsson K770I

ok so i have had the phone a while now and i have a problem with the phone , when i turn the phone on it work for about 8 second then , when it get a signal to the network provider the phone switchs it self off. the phone is pay as you go. so when i take the battery out then put it back in the phone it gives me 2 options FLIGHT MODE and NORMAL MODE. i go into filght mode and the phone works fine like new. when i go in to to normal mode then phone goes haywire.

dose anyone know what this could be.

thanks

all ideas welcome and apped