Default [Q] sony Xperia LT30at restarting

hi, i have a question. well my Sony Xperia LT30at randomly restarts itself. i didn't know this was the case when i purchased it from a Seller off Amazon. i thought it was a software issue and that an update to 4.1.2 would fix it but that didn't solve the problem.

A few months after i had updated my phone, while still experiencing the same restarting issues daily, i accidentally dropped it in a buck of water. it had a silicon case on and my headphones were connected and the phone did not go off when this happened. i immediately turned it off and opened it to remove the battery and let it dry. i had to replace the bottom mic because it stopped working after this and about a month later the battery was giving problems but everything else was working fine.

i recently purchased a new battery for it and whenever it gets below 75% the phone will turn itself off and the only way i can keep it on is by connecting it to a charger for a few minutes. at times it restarts immediately after i hung up a phone call

My question is, could this be a software problem and will updating it to 4.3 help fix the restarting?

please help