I'm sorry you're having problems with your text messaging. It sounds like your device is crashing when using the program. To resolve that issue, I'd recommend following the Android Crashing troubleshooting document.
Let me know if this helps!
Since you've had the phone have you done any kind of software update? If not, check out Software upgrade to Android 2.2 (Froyo) steps - myTouch 3G Slide, this will help you update your phone to latest software.
Funny... I've been having the EXACT same problems for 15 months. It started long before my warranty expired but tech support had me do a factory reset FOUR TIMES!! This would always fix the issue, temporarily. Two months after the warranty expired, the factory reset stopped working and the phone took a total dump. Now if I want to use my phone, ie..text, call, I have to take the battery out and back in which gives me about 15-20 minutes of use before the bs starts up again. Of course it doesn't matter to customer support that the issue started LONG before my warranty was expired, the only thing they're concerned with is that it IS expired now so they don't have any suggestions for me except to buy a new phone. It's kind of interesting to me that no one at tech support EVER heard of anyone else having this issue. Anyway, obviously I'm pretty irritated right now but did you ever find a solution to the problem?
This happened to me and nobody at support could explain it. Fortunately, I found an employee at the retail store near me who knew WTF. My SIM card was outdated and it would cause the tower to drop me after 15 mins no matter what, which often made the device freeze. She said it could also happen with a damaged or corrupted SIM card as well. She was able to transfer the data on my SIM card to a new one and I have not had the same trouble since. (The device still freezes once in a while, and I really need to replace it, but it's not a guaranteed freak-out anymore).
It might be worth a trip to your local T-mo store to ask them for a new SIM. HTH