So I've searched up this here board, but unfortunately came up empty. I have a 1990 Corolla DX. It's my baby (first car!) and I love it for the scrap metal people see it as (my high school's parking lot is dominated by BMW's, Lexus's, Benz's, etc.). Anyways, when I'm navigating the roadways of Virginia, minding my own business, sometimes when I shift from 3rd to 4th, or 4th to 5th, NOTHING HAPPENS.

The car decelerates to about 35-40 mph and stays there. This is quite a problem when I'm getting on the highway or leaving a toll plaza and this happens. Does anyone have some suggestions of things to check? I've already checked all of the fluid levels, so I know it's not that.
tl;dr- when I shift from 3rd to 4th, or 4th to 5th, my car decelerates to ~38mph instead of accelerating.
Some other issues I'd love to solve if anybody has any advice:
-Vinyl ceiling is sinking (how do you remove it?)
-bent frame (anyone know how much this repair costs?)
Thanks! I'll post some pictures of my stallion later. Gotta show off my gem.