I have noticed in the last two weeks that text messg on the Evo had been noticibly different. Letters/words totallly off the mark (I had this phone for 3 years - auto-correct and I have had our differences but never like these last two weeks). Today my screen locked up while in Yahoo Mail downloading a pix... it showed 3G lit up but no action, then I couldn't get it to turn off.... I also have had issues in the last 2-3 days where the phone would lock up in the phone mode, wouldn't accept any key selection nor any of the bottom buttons, had to shut it down with the power button. Any way to correct this mess? Any way to uninstall and then re-install OS?
It sounds like it may be a software issue with device. You may want to take your phone into a sprint service and repair store to get your phone tested or else you can just reset your device to factory default.
Note: A hard reset should only be performed after all other troubleshooting options have been exhausted or if you want to remove everything from your unit at the same time.
A hard reset removes all information from your device and returns it to a state just like it has never been used before. All contacts, messages, emails, 3rd party applications, etc. will need to be reinstalled after performing a hard reset.
To perform a hard reset, from the Home screen, press Menu and tap Settings > Privacy > Factory data reset. Follow the on-screen prompts to complete. This process will take 5-10 minutes.
To perform a hard reset using the device keys:
I'm assuming your referring to when typing a message, not receiving a message as you mention auto correct. Couple other things you can try are the Clear Cache/Data for the keyboard your using, whether it's the HTC Sense Keyboard or the Swype keyboard or another.
Search and locate both keyboards and Clear Cache/Data
If that doesn't work, try booting into Safe Mode and seeing if a 3rd party app is messing with the settings. Restart the phone and at First Boot screen, press and hold the Volume Down button until the phone is fully booted. You should feel a quick short vibrate followed by a much longer one toward the end of the boot.
If it's still wonky, do what Ray suggests above as that may be your only other option.