Im having a problem with my 4L80E, it works perfect in 1st hard shift to second, pulls good to about 5000 rpms drop to 4500 for a second then pulls hard to the shift, 3rd runs out perfect. The transmission has worked perfect shifts hard, but i havent figured out what is causing this. Ive got HPtuners and i thought it might be the converter locking up halfway through 2nd but ive got it where it doesnt lock at all unless its in 4th, unless there is something else in the tune that controls tranny or converter function that i missed...... Any ideas what to look for?

This is a 2000 GMC Sierra reg cab, 6.0, 4L80E with transgo shift kit, 3400 Fuddle lockup converter(it was the most expensive they had) Its running 12.3@105 with a 1.67 60ft on motor