All my apps (including messaging, email etc) were crashing, battery life was down to about 4 hours and during calls I would lose the earpiece for 5-10 second periods.
I had this issue suddenly effect my iphone 4s after about 5 months trouble free use. I intially replaced the phone with Apple Care who were more than helpful, this didn't however resolve the issue, as it ALL returned on my new phone.
I tried restoring the phone via my itunes backup but this didn't resolve any of the problems.
I have now fixed the issue though by restoring the phone 'as new' via itunes. I have had to sacrifice my photos, music, message history etc (on the phone) but I did back them up before the restore so I still have them to add at a later date.
I was a little worried that I'd lose all my contacts, but ALL my contacts have been restored via my email address book and Facebook contacts upon setting up the phone via the 'as new' process.