I had this same thing happen with my 89 Tristar 351 Indmar. I installed the electronic ignition, fired it up. It ran great, put my timing light on it and it was something like 24 maybe even a tad higher, I can't remember its been a couple of years. I tried to set it at 8, it didn't run well at all. I tried 2 different timing lights, same result. I had help from a couple of smart people from this site. It still wouldn't run very well at 8 btdc, and it was really hard to re-start.
Finally, with the advice of the same members on this site, I set it back to the original setting before I changed to the electronic ignition (24 or what ever the exact number is) I set it there locked it down, left it and ran it that way for 2 years and 100 hours. It runs great and purrs like a kitten! Not sure that we have the same issue, but it sounds very similar to what I went through.