I'm having a very similar problem. Car when started will go 20 minutes not a problem and then down to "3rd Fail Safe" and check engine light comes on. I bought a scanner and had codes: P0722, P0734, P1529.
P0722 I replaced the pulse generator 43.00 for the part pretty simple to do. Erased the code with my scanner. That issue is gone, a week later and never came back.
Still have issue P0734 and P1529. I believe 1529 is just the TCM telling the computer to turn the light on because "something" is not right. I believe the not right part is the P0734.
I know it isn't a mechanical problem because if you pull over and turn the car off and back on it's good right from the start for another 15 minutes.
I've found a couple different responses about similar problems on the web.
1. replace trans. solenoid
2. Replace kickdown servo.
3. Replace temp guage.
4. replace 4th gear and bearing.
5. Change fluid and filter. (Not sure when/if my sister in law ever did this. Car has 77,000 on it.)
Has anyone out there had any experience with this at all? Other than the individuals who have posted this same problem and have gotten no responses?