David, chiming in with what Taz said, if you're running an E38 originally equipped with a 4L60E (ECM says 4L60E as trans type) with a T42 running a 4L80E OS. Mismatch there could be causing issues.

Post your tune, I havent seen any Gen IVs aka E38s running with a 4L80E (doesnt mean they dont exist), so I might go back to running the T42 with a 4L60E OS then swap the few pins needed to control an 80E with a 60E OS.