722.6 Trans. ALL TAKE SYNTHETIC FLUILD. Low fluid level could be a problem. I have replace a lot of soleniod plates in the 722.6 Trans. It is the board that all of the shift soleniod connect to on top of the valve body. It also is where the input and output shaft speed sensors are located. If the board is losing contact with a speed sensor it will shift out of gear or go into what we call "limp home mode" second gear and will not shift up. If this is the case a code will be store in the ETC, the trans. control module.