onn

i have 845 with 1.8 GHz 256 mb, itz 3 yrs old till now had no problem but from 3 days it powers off after some 10 t0 20 mins,, it will not shut down but suddenly offs,, when it offs if i press power button again it will not restart,, but if i unplug and replug and onn, then it starts and powers...