Intermittent overdrive

You are correct. More than likely this is the torque converter clutch control circuit or the overdrive servo. I have rebuilt a lot of these transmissions and I would go with the the servo before anything else. Normally when the seal is cut it allows fluid to pass when it gets hot. This would explain the intermittent operation.

I included processes for both the valve body and servo. Unfortunately the only way to test this is to air check it. This means you need to remove the valve body and install an air check plate and if you don't have that then clearly you can't do it. I would just go with the servo. Let me know if you have questions.

Thank you for the response! I worked 35 years. as a field tech on heavy equipment, now retired. I have worked on many transmissions and hydrostatic drives in equipment, but never any of the automotive type transmissions. There is a transmission shop not far from me that I am going to take it to after the 1st of the year. They are well recommended and that is their only business. I will take my own advice that I use to give my customers, " If you don't know what you are doing, don't do it"! Thank you again, I just wanted to ask if I was thinking in the right direction.

Haha. Very true. With your experience, I am sure you could figure it out but I would not work on any heavy equipment even though I could figure it out. So I totally get it. Thanks for the update. Let us know what they find after you get it straightened out.