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Look I know you are not even going to want to hear this, but you probably already have a good idea, so here we go. There is no easy answer for this one. Normally a problem like this would be in the valve body, but you have been through the valve body. If it was mine I would be tempted to install another valve body. Sometimes valve body's just have problems you cant see. From there I would suspect a stuck governor might be part of the problem, its under the tail housing. I have found that many times when a transmission has overly strange symptoms that there is often more than one problem. I would also recommend that you get an ATSG manual and perform the air checks, and go over the valve body with their diagrams. The ATSG trans repair manuals are the gold standard that all trans pros use. I will post a link HERE for ATSG, I cant overstress how good these are they have gotten me out of dozens on tough auto trans problems. These manuals are far better than the factory repair manuals for transmissions. When it is all said and done you might have this trans laying out on a table looking for the problem, the air checks and inspecting the governor, or replacing the valve body might allow you to locate the problem without going through all of that. If you have any more questions I am here to help. Good luck, and Thanks for using Just Answer.

I hope this helps, 100% satisfaction is my goal. Just Answer is here to serve you. If the answer is not clear, please let me know what additional help you need, and I will assist you further. When you're satisfied, click on the green accept button. I don't get paid unless you do. A bonus and positive feedback are always appreciated, good luck and Thanks.