1st Gen Iphone deleting Text messages

I recently acquired a 4gb 1st gen iphone so I updated it with iphone 3.0. It works fine except when I receive a text message, I can reply to it and when I receive the reply to my reply, it clears out everything in my text messaging. It's really weird how it works but it has me beside myself. I have restored it 3 times and I'm still having the same issue. It seems like the Text Messaging App is crashing and deleting everything. Any suggestions?