Can't keep 3rd gear!
 

I have a tranny out of a 98 Trans AM wrecked car. I keep loosing third and fourth. I had a reputable guy rebuild mine locally with all the goodies and it has gone out on me twise. This time just after 60 miles. Do you think that he is doing something wrong with it or might there be a problem with the whole tranny being from a wrecked car?

There are alot of variables in a 4l60E that can cause 3-4 clutch failure,even with a good trans tech.Most likely your trans tech just has'nt found the true problem yet.There is always the possibility for a porosity leak in the case/and or pump casting,that you will never see.
As long as he is sticking by his work,and not trying to rape you,give him time.He should nail it down.