I've taken out my tranny 3 times for the same issue. Each time my 1-2 band is shot. It has drove Vince Janis nuts for 2 years. We have tried everything. Let me know what you find.
I dropped the car off a Janis Transmissions. He has replaced just about everything. In hashing it out...thought is that maybe the valve body is not perfectly flat and maybe be warped. This would cause my issue. He will be adding some other items and converting it over to non lock at the same time, so hopefully it cures the issue. I know Vinny has built 100's of bad ass trannys but this problem has jerked him off plenty of times.