E78 throttle fluctuation with constant pedal position at constant speed

[motoxxxman]

I'm totally stumped with this stupid issue.

e78 running a L96 motor from 2013 2500HD 4wd 6.0L gas
t42 running a 4L60E from 2013 1500 2wd 4.8L gas, has a mild shift kit and corvette servo installed
no other modules installed, its in a pristine 1981 cadillac coupe deville
TB, pedal, and intake manifold are from a 2013 6.0L Caprice. all diagnostics for both parts pass at all times.

yes, the two modules and engine/tranny combo work fine together, though it did require a lot of custom wiring, including splicing the speed sensor wires together so the one sensor can feed both modules.

the issue I cant seem to resolve is whenever the throttle pedal is between 1%-30%, the throttlebody constantly fluctuates between 10%-20% causing a surging. it does not seem to do it in 1st, 2nd, or 3rd gear, but it does do it in park/neutral and in 4th gear at pretty much any speed. it's most pronounced below 10% throttle pedal position. And it only does it while cruising at constant speed or at very light accel or decel.

I have played with practically every torque table, tcc table, and spark table, and I simply cant get it to go away. after making some major changes to the torque tables, I was able to smooth it out a lot in those higher pedal ranges (15%-30%), but it made it worse in the 1%-8% pedal position range.

any ideas?