my trans when full throttle will sit at highest rpm for a few secs before shifting. what could be the problem?

just recently it clunks when shifting in to reverse. i know its the gear and it seems like there is to much space between the mating surfaces and so it clunks.

i know the fluid is good as i had to do the pass. axle seal that was and still is leaking.
i bought a seal from honda this time instead of napa and will be replacing it here shortly.
the fluid is not low as i check it often from the leak.

the trans was replaced with a used one by the old owner.
it came out of a 05 per the VIN.
not sure how many miles are on it but the car has 142,xxx.


any help would be great.
Thx,
Zach