transmission problem

I have a 1997 f350 diesel with automatic trans. Was pulling a wagon and trans went out. It felt like i had put it in neutral. Pulled over and it would not move. After a little while it went into gear. Started home and it kept shifting back and forth between first and second. The shifts were very abrupt and constant. Pulled trans and had it rebuilt. Reinstalled and still have same problem upshifting and downshifting. Code shows p0603. Had chip installed but removed it and still same code. Is it in the computer? Does trans have a computer of its own?
Thanks
Rick