Phone randomly restarts after water immersion and being dried in rice

My daughter had this iPhone 3GS that got dropped in some water. She dried it in rice for 4 days and all was good. After a month or two it started restartting randomly. After it got so bad she couldn't use it for business conversations she bought a new one (ouch). She sent it to me so I could pull it apart and see if i could fix it as a backup. I took it apart and have found no sign of water damage under a 10x loop except maybe some possible corrosion on one of the vibrator motor springs but that is probably a stretch. The battery contacts are clean and I do not believe she was having battery problems. She had attributed all issues to the "water immersion" event. I see several people have posted issues about battery comparability but this is the original battery. She has had several iPhones and was prompt with the dry out procedure and I am thinking that it is possible that water never got into the phone. Any ideas, I can't take it back to the apple store as I removed the don't remove sticker