From attempting to analyze this thread, I see two probable causes for this behavior.
1) Too quick on the trigger - don't wait for message interface to catch up when beginning a text, resulting in sending to the wrong person after the interface finally displays the real recipient. Seems like waiting and verifying the recipient before starting to type would "fix" this.
2) Leaving draft messages in your draft folder, resulting in the phone "resuming" an old message to someone else by mistake. This might be cured by making sure you never have any "draft" messages. You can get a "draft" by mistake by simply starting to reply then changing your mind and using the [Return] arrow to back out. This automatically (and maybe unintentionally) saves a Draft. If you have a draft message you can tell by Messages | Menu - if you see Draft, you have one or more and should delete them. If you do not have any drafts, you will not see a Draft folder at all.
Just my thoughts. I have never had this issue. But, I keep my drafts deleted and I'm a slow texter (the interface is faster than I am anyway)
-Frank