I had the same problem. my old sony tv had geometery pots on the pcb, so i tweeeked them to sort the overscan, but my new tv has bugger-all pots to fiddle with. if you have a SONY tv purchased in the last 10 years, you could try entering the service menu. Turn on the tv by the pb on the front on the tv, go into standby (from the remote), on-screen disply (on your remote), channel 5 (on your remote), VOLUME + , and finally TV mode on your remote (the one that takes you to watching tv after looking at teletext). Your tv should now come on with "TT" displayed in the top rhs, press menu on the remote and navigate with the coloured buttons....WHEN YOU'RE DONE, PRESS 00 (ZERO,ZERO) TO EXIT
THIS IS A PRO SERVICE MENU SO DONT BLAME ME IF YOU SERIOUSLY FRAG YOUR TV....USE AT YOUR OWN PERIL.