This has all the signs of low oil level in the trans....add a quart of ATF and try again....
I have seen aftermarket dipstick tubes and dipsticks be out of specs resulting in issues.
I have also seen an ill fitting trans filter creating an air leak into the pump housing..causing pressure issues....slow gear take up, poor shifting etc...
I suggest you change the filter and make sure there is a O ring on the tip of the feed tube that fits into the pump....
Does this trans have an aftermarket deep pan? The original pan has a dimple on the bottom of the pan to keep the filter in contact with the pump....plus there is a bolt thru the filter body to retain the filter...
Finally, make sure the detent comb is not holding the manual valve in a misplaced position....while the pan is off, check that the drive position of the gear lever equates to the detent spring being in the middle of the D comb position....
