hey scotty, love the your channel. what do u think could of cause the problem in the first place, what parts could have been damage?

wondering if you could help me, i have 2002 mazda familia sport 20 automatic with over 140000 miles on it, me and gf recent got the car, we love it...however we start noticing in the morning the car was taking a while to move off when we put it in Drive it will just rev out and wont move, it will take a few minutes before it can engaged but after would drive normally and reverse gear still work like new . we took it to her father who is an mechanic, he check it out and realize someone had work on the transmission before, he serviced it then use some brakes fluid, it work like a charm for about 1 month. unfortunately it start giving the same problem even worse now, it takes even longer to select and when it does u actually feel it. kick in it really scary and other time it will fail to change gear and some times it will just start rev out while driving, and wont move, it not work again until it cool down. her dad isnt in the country right now, we him told he say most likely we might have get a new transmission, we live in the Caribbean