Q: Rebuilt for 4.3

Brand new rebuilt 4.3 everything in the motor is new except for the block it's all stock had everything set up timing marks inside outside distributor turn it ran about 6 hours not straight off and on the break in the new cam rollers and did some adjustments some wiring and stuff like that the last time I started it up fine ran for about 20 minutes went to get in and move it put it in gear and it died and it would not start back up started checking the obvious crank sensor cam sensor Spark timing mass air flow air temp sensor throttle position sensor idle control sensor started kicking and bucking like it was 180 out pulled apart a the front of the motor to double check the timing timing marks are lined up timing marks on the outside of the timing chain cover lines up distributor lines up put the front of the motor back together still no fire kicks and bucks like it spark timing is out of whack guess my question would be what else would cause that ?
My car has an automatic transmission.