Auto Trans prob;lem with 2nd to 3rd shift

I have a '89 Lebaron with a 413 auto trans with a problem. When it makes the 2nd to 3rd shift it feels like it downshifts. It does go into third gear a split second later. I had other cars where they slip but not like this. Doesn't matter if I barely step on the gas or not. If my tach worked maybe I could tell more. Car just hit hundred thousand miles and I have changed the fluid & filter last year. I may put five thousand miles on it a year. Is this an adjustment or a fluid change ? Thanks.

Is it flaring on 2-3 shift? (rpm goes up for a second) or slowing the the car down for a second? (bad 2-3 overlap)

I found the problem. The line pressure cable came off the throttle body.