Chasing major gremlin, hope this fixes it!

B3422w5 said

Sounds like the exact problem i had years ago with a stroker smallblock. Missed at the top of second and third gear going down the track under load.
Chased my tail nearly 1/2 a season
Turned out to be the spark box. In my case it was a 6al at the time.

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That's about the only thing I haven't replaced. Several calls to MSD and they are certain is isn't their 6Hemi controller. Come to think of it, I haven't replaced the ignition wiring harness either, but if they fire fine upto 7500rpm under light load I don't see how that could be it.