tranny problems

[94LSEG]

ok when i first bought my car it was havin problems shifting after 6 grand and i was told it was the pressure plate now i changed the entire tranny that i had in another car i had and it was good in my other car and now its still doin the same thing and we changed the clutch slave cylinder so lmk what the hell is goin on cause i have no idea and i dont really think its my tranny cause it was good in my other car. it just slips out of gear at 6000

[94LSEG]

and its in every gear well atleast 1st and 2nd cause thats the only ones i have done it in