1994 Chrysler Le Baron transmission down shifts on its own

Good morning gentleman. I have a 94 chrysler lebaron conv. With a 3.0l automatic. My question is this. After warm up this car will down shift to second gear and stay there until it is shut off and restarted. Ive already replaced the ecm. Input and output sensers along with a filter/ fluid change. The woman this car belongs to is elderly and wants this repaired, I personaly can not afford to put any of my own income into it. Short of tearing into the wiring harness even further I dont know of any other options. Any light on this subject would be greatly appreciated.