First check your oil. when was it changed last what colour is it? It should be red. Badly contaminated oil in an auto will lead to slip.
I had a problem perhaps similar to yours on my 91'. The transmission shop said it needed a rebuild but they would not test anything except to road test and it never slipped during the road test. after living with the occasional slip into neutral for years a mechanic finally diagnosed the problem as one or more failing solenoids. The expense of replacing a solenoid is prohibitive and perhaps more than just buying a used tranny with low miles and swapping. I got one with a year guarantee for four hundred but have not installed it yet. Sometimes solenoids will work abnormally with a bad contact or may stick due to corrosion. It is worth cleaning and checking the contacts first.