This topic is covered extensively here
http://www.automotiveforums.com/vbul...d.php?t=920671 Basically, you should install a shift kit and that will probably solve your issue. Here is the shift kit that I used on my car, I have had it in there about 2 years without any more problems.
http://item.mobileweb.ebay.com/viewi...assified=false hopefully that link works. If not, just search for "transgo 4t65e shift kit". It's about 60 dollars on Ebay.