Runs fine for 15 seconds then dies and won’t start

[El Campo]

97 Chevy with the vortec V6. Truck sat for few years and then got running again. Due to sitting for so long etc went ahead and put the MPI system on to replace the spider. Cleaned out old gas and everything. Now when it starts up it will run perfectly fine to 3000 rpms for about 15-30 seconds and then it dies. If u try to restart it immediately it will backfire and cough but will not start. Come back 15 minutes later and fires right up and runs good for another 15 seconds and then dies. Any ideas???
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