TH 400 governor issue

[MAC71]

I have a 69 C20 that after buying it and getting it running and on the road the TH 400 transmission failed after a few days of driving. Pulled it and rebuilt it and everything works well with the exception of the governor. It will hang up and it will not up shift out of first gear. I pull the governor out clean it with brake parts cleaner and compressed air and re install it and it works fine and I will get 400-600 miles on it and it hangs up again. It has done this so far 4 times, easy fix but it always seems to happen on the way to work.
The valve in the governor moves smoothly and no abnormal ware, and it fits well in the boar in the case.

What am I missing here?