Trans help

[Showpanther]

Hey guys.
My 97 F350 dually has an automatic trans. I have had 2 new transmissions put in after overdrive went out on both ( The second under warrenty luckly)
Now the transmission will not shift down when I stop at a light or something.
I can manually shift and it is OK. Jasper said it was probably trash in a valve and to service it and add an additive which I did. After a week it is no better. My question is could it be a sensor or something else electrical.
Thanks for your help.

[WD40]

You only need to know one person when it comes to working on a auto, It's all on his web page.

http://www.brianstruckshop.com/index.html

[Showpanther]

Thanks WD40
I sure hope I dont have to go with another trans yet. But that will be the way to go if it comes to that. I still cant help but think it is a sensor or something. I hope I'm not looking for a miracle, I need a break!