I would recommend trying a different sms app. I use handcent. Deals with mms a lot better than stock and has lots of customization and usability options. This should help. if not, it could be another program behaving irregularly, so I would try a hard reset. Then try putting apps back on one by one and keep to market or high rated apps only. Try using handcent and do not use a taskiller. These tend to have negative side effects after a while. if none of this works, take the phone back or call costumer service. Hope this helps.
