1987 Nissan Truck engine problem

1987 Nissan Hard body 4x4 3.0 mileage: 244,000. I am a mechanic and this has 4 others baffled. I can go out in the morning and start the truck and it runs great. When I go into gear and drive it dies. I loses spark and dumps fuel at the same time. Now you have to wait about 2 hrs to start it again. It has a new distributor, coil, and cap/rotor. I did the tap test on the computer. If you crank it for a while it will finally spark. So can you help. Thanks.