I was going to ask the same question and then saw this post. I have had the same problem with the port engine. My mechanic re-built the carb and it still happend then we change the relay and still the same problem. We then changed the coil and it worked great. We took her out the next day and it ran awesome. After spending the day on the hook she would not start. kept cranking and would start for a sec or two then died. I could smell fuel coming from bilge. Like it was geting too much fuel. Limped home on one engine mechanic coming tomorrow. He is in the dark. Says it could be the ignition module. This is driving me nuts.....
I am sure your mechanic will be there by the time you read this but here is what happened to me in the past.
Had a 7.4 in a squirt boat- did a tune up it ran great for day or so then it just quit running. It would turn over and run but as soon as I let the key go it would stall. Much cussing and fussing I finally found there was a wire broke going to the coil. used the key to strip it and wrapped on the post and zoom zoom got her home and fixed it right. Mind you this was not a HEI ignition but you could still get the same type of problem with it. Hope this helps, just check your coil power wires no matter which ignition youu have.