Stalls after 20 seconds

Hi I have an '89 c2500 5.7 that will run for approximately 20 seconds then die. After it dies you have to turn the key off then on again and it will run for 20 seconds again then stall. The only way to make it run is by bypassing the fuel module. It runs in open loop always. I'm suspecting a problem with the ECM. Please help it's driving me nuts!

check pickup coil in dist and ignition module in dist.

check pickup coil in dist and ignition module in dist.
thanks I didtn't think of that.