Page 1 of stupid sony ericsson v640i

hi i recently ordered a sony ericsson v640i from vodafone but had to send it back as i would turn it on and about 2 mins later the screen would go blank but the lights would be on.Had a new one sent today and thought i was chargeing it but when i turn it on it will say sony ericsson then the screen will go blank and it will just do a short vibrate every few seconds.can anyone help me with what is going on?seems strange two phones doing the same sort of thing