Back in the day, my buddy had a Dart Sport 340 that did the same thing. Crank it's guts out....release the key and it would start. It ended up being the pickup module in the distributor. Fast forward to around 1989. I had a Dart Swinger with a 318, and it started doing the same thing. I replaced the pickup and star wheel in the distributor and it fixed it. Try replacing those parts, and use a brass feeler guage to set the gap. Good luck.