Venture Transmission

1999 Chevy Venture van, 3100 V6 automatic transmission. Has issues with the shifting. When its cold it will not go into gear, just act like its in nuetral. When it warms up it will go into gear fine and pull and hold fine, but it just left me stranded sunday night. While I was driving, it just started slipping or acting like it was in nuetral and wouldn't move and I had to ahve it towed home. The next day I started it up and it wouldn't move, while I was checking the fluid level I left it running so it was warming up. The fluid level is a bit over full and has a slight burnt smell but not to strong of a smell. After the van warmed up again the trans worked fine and pulled and held fine. I even went so far as to put it in gear and hold the brake and try to go to see if it would slip but it held and pulled still with no problem. Help