let me see if i can give you some help, im a tranny mechanic so who knows

first off, if its got over 75000 miles and its never been serviced, dont change the tranny fluid, youll be better of driving it till it dies. now, what your describing is called a shift flare, not slipping. whats happening is the pressure to the clutch gets ramped up progressively to bring it on smoothly, however, yours is doing it too soft and is allowing it to flare before it fully engages. several things can cause this. a fluid level that is too low OR too high will cause this because if the fluid level is incorrect it causes the pump to cavitate creating air bubbles in the fluid, which makes main pressure erratic. another thing that can cause it would be a sticking valve in the valve body. my suggestion is this. see how long its been since its been serviced, if its been less than 75000 miles then change filter and fluid and make damn sure the fluid level is correct. regardless of whether you service it or not, drop the tranny pan and see if there is any clutch debris in the pan. if the pan is full of debris then most likely the clutches are burnt or there is a bearing failure (if its metal in the pan) and its causing the shift valves to stick.

let me know what you find.