Slant 6 Turbo 68Dart Project

I'm going to assume it's stock unless someone replaced it in the past (which I doubt) but it seems others were not having this issue with the stock pickup, unless they replaced it with a different one I guess.

The only other thing I can think of now, is that the timing is set to 8* adv, with 10* mechanical adv, and then the vacuum advance that goes up a bit higher, but under boost, the vacuum advance shuts off, and I'd guess the car stays at 18* without moving forward or back (not sure if that's true) could this cause a limiter like that?