TCM problem most likely, but could still be the converter I suppose if you drive "spiritedly":
http://www.hotrod.com/techarticles/g...e/viewall.html - "old article, but good stuf ... "
6L80 Six-Speed Automatic:
This new-gen GM trans is so sophisticated that it has a separate computer (TCM or transmission control module). Incorrect TCM tuning causes more problems than hard-parts failures.
TCM calibration: Reprogramming is required when changing engine rpm range, shift points, or rearend gear ratio. A significant power change increases engine acceleration rate, which changes the amount of trans slip; both impact when the shift needs to occur compared with when the TCM calculates it should occur.
TAPshift lag: When enabled, on-demand manual gear changes are made via steering wheel paddles. TAPshift permits holding a specific gear around a road course without fear that the trans might upshift or downshift during a turn and destabilize the car. It also works well for a burnout, but input lag creates straight-line acceleration issues; for drag racing, reprogram to upshift automatically at a desired shift rpm.
Torque management damage: Disabling torque management completely can cause failures. LPE says, "With a clutch-to-clutch trans like the 6L80, some torque management is needed to reduce the torque momentarily and allow time for the clutch for the desired gear to engage."