the transmission in my 95 pontiac sunfire slips sometimes but not others.

i would check the oil level in the box as it feeds the torque converter. with not enough oil once it has spread around and can't feed enough to create drive it will cause the symptoms you are describing it also explains why when you shut it off for a bit it started to move again as it has all the oil in the bottom and was able to pick up enough to make the converter work again.

i would rcomend a fluid and filter change. most people tend to neglect that since the transmission is an automatic and most people tend to forget about changing transmission filters. Its the same as any filter. if you dont change it, it may get clogged over time and transmission fluid may not circulate properly. You can go to most tranny shops for the parts and most likely will have them in stock.