Transmission slips

I have a 1990 dodge ram van B250 sitting on my driveway for 6 monts and try to use it but my tranny no work I change the filter was black change the fluid and works fine I took it to work 1 hr drive not problem when I get there when I come back from work 5 mins before my house my tranny not working again so what I do is turn off my van and turn back on and works for a lil bit so when the tranny is cool works but get hot and slips told me whats wrong with my tranny