IPhone suddenly turns off constantly, can I fix this?

Ok so I have a second hand iPhone 3Gs that I won't lie. Is pretty battered. The person that had it before me dropped it in water and generally didn't take good care of it however I have been using it now for about 9 months and it has fine until now. I slipped over a week ago with my phone in my hand and like whenever it gets a small bump it turned off. This time it kept constantly turning off (as in sudden black screen) and when i turn it on it sometimes used to constantly show the apple logo then go black then show the logo as if it was trying to turn on but failed each time. Basically my phone now stays on and rarely turns off when on charge but whenever i take it off there's a very high chance it will turn off when sending a text or overall use (But it stays on when its just the lock screen?). I think something has caused the battery and/or wires to come loose but I really don't know. Any help or suggestions would be appreciated! Thanks.

Added (1). I can't afford a new iPhone right now. I wouldn't want to buy another 3Gs anyway. I would buy an iPhone 4S but that's not really an option for me right now.

You'd better try a new one, it is not expensive.

It seems your ipone 3 Gs will past in a while. Better you buy a new device.

It may just be the battery or battery contacts. Check that the contacts on the battery are clean and that the connections on the phone are not corroded from the soaking or damaged from the fall. The battery may have an indicator on it to signify if it has suffered water damage. It's not always reliable and may turn red over time anyway on an older battery but it's worth a check although not conclusive.

If you have another compatible phone you could try swapping batteries with that. If the problem moves to the other phone and your 'battered' 3GS is okay then you most likely need a new battery. If the problem persists with the 'battered' 3GS, it still may just be the battery contacts or an internal connection related to power. I'm not sure where you are but a quick web search shows batteries, some original Apple, available in the UK for under £10.It may be worth that kind of investment to save the phone.

I dropped a Palm Pilot once and the internal battery connector became detached from the circuit board. It was fiddly to resolder being so small but worked fine afterwards. If you're not up to doing a repair yourself but you've all but written the device off, you could find a friend with some very fine tools and very sharp eyes!