signal problem with k800i

hey guys new to the forum so forgive me if iv posted in wrong place, anyway my problem is that i recently bought a sony k800i from a guy i know, it had been a present for his girlfriend and they broke up so i got it dirt cheap as it had been lying in his drawer for a couple of years. anyways when i first turned it on it seemed to work fine apart from a poor signal constantly only one bar, when i take the phone into open areas, sometimes i can get a 3g signal but indoors or in a secluded area, either no network coverage or emergency calls only. i flashed the firmware in the hope it was a problem on that front after reading through some forums but to no avail, the warranty is out and i was hoping to get it fixed with minmal fuss, any ideas?

i forgot to mention that for some reason it was working with a 3rd party battery when i bought it and i have now put in a sony ericsson battery which seems to improve the regularity of me getting a signal but still only one bar or nada in most areas

Well then, i guess this is hardware problem, mostly antenna line.