2008 Mazda 3 AT Light

Hope you can help. I have a 2008 Mazda 3. Back in April I was driving and all the sudden the AT and CE lights came on and my car shifted down to third. I pulled over and turned my car off then back on and everything went away. The next day it happened again. I took it to Autozone and got the code read. It said my solenoid was sticking and they suggested getting it replaced, but it may or may not fix the problem. We ended up getting a used transmission, it had 30,000 miles on it. We had it installed. The day after driving it home, the EXACT same thing happened. We took it back to the transmission place and they pulled the same code. So thinking we had a faulty used transmission, we had the solenoids replaced. They call, we go pick it up. I get less than a MILE away and same thing. AT, CE lights come on and it won't shift out of third. We turn around and they run the codes again. Same ones. Obviously it's not the solenoids. Fearing it was the transmission people, we have taken the car somewhere else. But I am curious what your opinion is. I've done hours of research on this particular problem. It seems as though there are thousands of Mazda owners with this same problem. I am seeing no definitive answer or solution.