Having a problem with transmission slipping outta gear?

I have a 93 mazda protoge with an automatic transmission and when I drive on trips over 50 miles it seems to slip out of overdrive and if I slow to under 15m.p.h. it acts like it doesn't have 1st or second gear.. The hold button lights up on the dash and if I turn the car off for a bit it acts like there is nothing wrong till i go about 50 miles nonstop, then the process is the same thing..