That doesn't mean it's not the ignition system. Is it the same CDI box the whole time?
I had that same problem in my car nearly 20 years ago. One run it would go 12.50 and then it would run 11.90, it was either or, nothing in between. It started when I swapped from my old 6AL to upgrade to a Digital 6+ that had a timing retard for nitrous. I was tired of moving the timing manually to run the giggle juice. At the same time I changed the trans to an OD with a transbrake so it was better to drive to the track on the highway, part of me wondered if it was the trans. This was back on the carb with the iron headed 388 SBC, no delay box.
Of course I changed the whole car around after so I never confirmed that's what it was, but I wondered. Still have the same trans, everything else is different.
I'll never forget I went to do some testing back in the day, and there was a huge "Michigan State Finals" bracket race they called it that weekend I was unaware of. Was only an extra $30 to enter, I thought, what the hell, I'll enter the race for the first time ever, I'll get at least 1 more run on this busy day! ha ha
Car would run 11.90, 1st round I won, felt good, was not a stellar run, had a slow .028 light or something, but enough to win that round.
2nd round I get up there, guy goes red against, me, won again. But the car was slow, 12.50, changed spark plugs and double checked timing and everything. dialed 11.90 because sometimes driving the car to the track with the race carb would foul plugs, and went back up there for 3rd round, big mistake. As soon as the car took off I knew it was not fixed, I could feel it was slow, I felt like I was a sitting duck. Then around 1000 ft the opponent flew past me and broke out, I was thinking, I left a huge window and he took it all and then some, but I'll take it!
Went back to the drawing board, took the carb apart, found a chunk of something in the needle and seat!
Went back up for 4th round, and I was .004 on the tree and ran dead on with a 8 for a 12 pack, won that round and finally felt like I deserved it. Thought the problem was fixed.
5th round I was feeling good, just had a .004 light, and the car ran what it was supposed to previous round. So I dialed the 11.90. This time, guy went red, but my car was slow again, man I was lucky!
So 6th round I go park in the staging lanes, and I was pacing, trying to figure out what to try next. Track official says, what's on your mind? I said, I'm just not sure what car is going to show up..." He says, "this is all of them, there are only 4 left". I said, no I mean my car, it seems to have a mind of its own tonight... he laughed.
Then I thought, WTF, how did I make it down to the semifinal on my first race ever with a car this inconsistent! At this point it was getting dark. And I thought, I am going to unplug the fans, maybe they are causing an electrical issue. So I unplugged the little thermal switch so they would only turn on manually, and I left them off during the run, which I should of been doing the whole time for consistency. I stupidly dialed the 11.90 time thinking I wasn't good enough to judge the finish line. However, if the car was slow I would of locked myself out of winning pretty much, rookie move.
I let go of the button on the bottom bulb, I knew I was late. I thought, ****! But the car was running, running good! I thought, go baby go! At about the finish line I see the opponent creeping up on me, and then tap the brakes, I knew immediately I screwed up, in all the ruckus I forgot to dial back as the air temp dropped and I broke out running an 11.89 or something, car ran the fastest time of that summer day, but I lost at 4 cars, I won some money at least, paid for the day of fun is all. The guy that beat me won the whole race.
Was a good time even with the spontaneous car. ha ha