M1009 transmission shift up problem

[phantom2000]

http://www.steelsoldiers.com/showthread.php?37321-HMMWV-Transmission-Trouble
I have a 1986 M1009 with the TH400 and I occasionally have the same problem as the linked thread mostly after a warm start. Other times it shifts right and smooth.
I did drain-refill of the transmission fluid a few months ago and added Valvoline Maxlife. The fluid that came out of the pan was clean without any particles.
Has anyone experienced a similar problem?

 

[cpf240]

Have you read the CUCV wiki? Most likely there is a vacuum leak. "If its electrical, clean it. If its rubber, replace it"