Help, C36 stumbling after 1-2 shift

[BongC36]

Gentlemen, My 1995 C36 recently had its defective engine wiring harness replaced by an indy. Everything seemed fine until a couple of days later when I had a chance to do some italian tuning.

Whereas before, my 1-2 shifts at WOT were firm, crisp and the RPM climb uninterrupted, now the engine RPM drops immediately upon the 1-2 shift, hesitates for about 1 second, then resumes its climb up the power band. Disconcerting to say the least. 2-3 shift is unaffected so far.

My mechanic is sure he connected the new wiring harness corrrectly, and no other changes were made to the car at the time.

Perhaps some fellow members can help me diagnose this?

Many thanks

Bong