I had the same shuttle shift that you guys were describing and in the end it ended up being the transducer and pressure sensor on the valve body of my tranny. I went to these guys and got both parts which are an upgrade from borg warner and it immediately fixed the issue. NO MORE SHUTTLE SHIFTING!!! And the parts were cheap, and they only took about 20 min to replace, very easy fix.
the link to both parts are below.
http://www.cascadetransmissionparts....cer2000up.aspx
http://www.cascadetransmissionparts....enoidborg.aspx