IPhone – How do disable control by iPhone headphones

It certainly sounds like the headphones are causing this. It's likely that moisture is eventually getting into the in-line switch/microphone module which is bridging the switch terminals and sending spurious signals to the phone.

However, I'd try the following:

• Check for liquid damage inside the headphone socket: Hold your phone up to the light so that you can see down inside of the headphone port. There's a small indicator at the bottom of the port which will have turned a red/pink colour if liquid has come into contact with it. If positive, this could indicate that the phone's headphone socket may (also) be at fault.

• Obtain another pair of the same Apple headphones and see if the issue occurs again.

I had this issue with my iPhone 4. Turns out some debris got in the headphone jack. As the 3GS is an older device, the contacts may be dirty.

Try taking a cotton swab with most of the cotton removed and dipping it in rubbing alcohol. While it's off, clean the sides of the headphone port on the 3GS.

Let it sit for five minutes and turn it on. See if that helps.

Another thing you can do is quickly and forcefully insert and remove the headphones in question to (hopefully) clear anything blocking the contacts.

Personally, I'd try the first option first.