Hi guys and girls I'm a newbie

Soooo brought my own very stylish V70 08 and all is amazing but problem with the built in sat nav (also known as the RTI) oohhh I'm good lol well first problem was screen was coming up but full of lines (could hear the man talking) so brought a new refurbished unit as thought it was the screen. Unit arrived today fitted it and woohoo I have a screen and a map stating my location (although very old) and then screen goes down and turns off, leave it 20 mins go back out screen comes up, maps load can go through settings and then screen goes down again? Can anyone shed any light please? Thankyou