transmition trouble B2

hi all,before sorry for my bad english, i'm frenchi. i have a Bronco II 2.9 L with A4LD trans. For starting, i need to climb at 4000 t/m for that the trans passed to the 2; after she climb normally in 3 , an in overdrive, but she have difficult to stay up overdrive if a drive slow, she have troubleshotting http://broncoii.org/forum/Smileys/b2/frown_2.gif; she jump in drive 1 or 2 sec an she rejump in OD, she do not if a drive quickly ( more than 80 miles/h). http://broncoii.org/forum/Smileys/b2/004[1].gif
Have you one idee from this trouble
Jero

Welcome to AF!

From your description it sounds like it doesn't want to stay in gear. There is a vacuum diaphragm on the side of the transmission called the modulator valve. It is run off of vacuum from the engine. If the diaphragm gets a tear or hole in it it won't shift properly.
Also, if the diaphragm in the modulator valve is torn or has a hole in it, it will pull transmission fluid from the transmission into the intake of the engine. Have you noticed the transmission being low on fluid? Does your engine blow out blue smoke? If either of these 2 questions is yes, then the modulator valve on the side of the transmission needs to be replaced. It is just a metal disc with a rubber hose attatched to it. You're english is fairly good. Thanks for visiting us here on AF.