I am having a problem with my tranny when I get up to 40mph it will jump out of gear in drive and over drive 2nd and first and reverese work fine have replaced the transmission selinod in it but still having same problem shifts good until I get up to 40mph the truck is a 1996 4x4 sonma highrider automatic with over drive.  Can some one help me figure out hats going on?  Have asked but some said it might be speed sensor but than they said it might be some internal problems.