It sounds like you need to make use of SMS backup and Restore, and back up your SMS every month. I have heard of numerous people complaining that their SMS all of a sudden get deleted, and almost every time they have disabled automatic deletion.
It sounds like you overwhelm the system and saving large sets of SMS is killing the phone's memory, until it hiccups and throws everything out and tries to start fresh - but it corrupts the data base as well. Check to see if you can add a new contact and then check from your PC is your Google Account has that new account - if not, it may be that your contact database on the phone is corrupted, and you'll need to make a backup of your contacts in your Google account via the PC, and then delete the account in your phone, and add it in again (I only say making a backup on the PC / Google side b/c if it is messed up on your phone, then any manipulation you do on the phone may corrupt the database on Google - so better to back that up just in case)....
Report back if a new contact works, and we'll go from there.