C6 no third when warm

[Chuck2422]

My recently rebuilt C6 in my 69 Ranger SWB that is attached to my 360 will shift normally until 10 minutes into driving when it will start shifting later in the RPM range then not shift into 3rd at all. I can shift it manually into third no problem for the next 10 minutes of town driving but after that it will no shift into third at all. It acts like there is no gear there. I will have to put it back into 2nd and try a few times to go to third. If I leave it on 3rd it will feel like it is in neutral, no pulling or engagement. If I let it sit and cool it will start shifting normally again. I have adjusted the modulator out multiple turns for earlier shifts, checked vacuum line and replaced that just to be safe with the same results. I am not running a trans cooler. I was told it could be a dirty or malfunctioning governor. Does anyone have ideas?

[Nitekruizer]

If the trans was rebuilt by a shop and is still on warranty take it back to them and let them drive it. If not, someone who knows transmissions will chime in soon.

[Chuck2422]

I finally had time to return my truck back to tranny shop. After about 75 miles and a month of trying to figure out the issue it was valve body. They are replacing it for free.