GM transmission, the 4L80 E unable to select 1, 3 or 4th..

This is a new crate transmission from GM, as of last September, once installed, my problem was the exact same - the old transmission was still good! After much fiddling around, and unsucesful tryout drives with the brand new tranny installed, "something" changed and we got all the gears. After three more episodes during which we traveled several thousand miles, the problem has recurred. A mechanic in Marbrorough, Mass. recomended checking the instrument array for problems.