this has been happening for a while but it has been especially bad the last 2 weeks. I usually keep ongoing conversations open and lately they have been deleting or closing. It doesn't prompt me to close them - it just closes and I lose the entire convo.
That sounds fine for now, but I am still thinking you were low on memory. What you describe sounds like the typical symptons. If you are low on memory, the device will start deleting messages (email, sms, call logs) to free up more space. Usually this happens if there is less than 10,000,000 bytes free.