1990 Dodge Caravan Ignition shut down

Recently, my caravan stopped running while I was sitting in it. When it failed to fire up, I checked the fuel pump (good), then went to check spark - nothing. Timing belt good, so I swapped out coil pack with spare I had. Started, but only after disconnecting battery. Ran short time, stalled again. Swapped computer, with spare. Same problem. Van will run after computer cleared but something causing the computer to shut it down. It is equipped with an after market command start I have never used, but it never caused a problem before. I hope you are fammiliar with this problem, and thank you for any light you can shed on this.