I ve got the same setup built trans/1058/ stock injectors and mines got a very similar feeling when it shifts from 3rd to 4th while the converter is locking right at the top of third. It ll also bark the tires once lockup occurs in certain situations.
Yeah I rarely notice it anymore since I ve been using Ben's box. I just assumed it's the nature of the beast in this converter and never really worried about it.
When I was running a 1058 with a single 76 it would shift into 5th. Converter would lock and drop rpms causing it to down shift. Got trans tuning that bumped up shift points a little and never had anymore issues.