Backpressure on a turbo’d car does mean something, especially depending on how far down the exhaust from the turbo your exhaust wastegate gets its feed from, but that’s a separate discussion. Like I said before, this is the least likely of the options.
My car produced similar problems a couple months ago. It turned out my Detonation sensor was on its way out. I would suggest it here but there’s no Detonation sensor on the S12. I fixed it in an hour with the Detonation sensor off of Alex’s last RB motor.