Transmission on my 1994SE 6cyl

Transmission on my 1994SE 6cyl. Does not always work after a period of idling or stopping in midst of a long drive. Sometimes it will work in 1st but no higher. Other times if I stop and put car into park and re-start it, it will eventually gear up to drive. Trans. Was serviced 3or4 years ago. It was looked at by a mechanic 1 and a half years ago but it worked fine for mechanic and didn't read on his lap-top as having anything wrong. Car sat in a garage for a number of years prior to my purchase of it approx. 4 years ago. It sat out 4 months during past winter and I have only just resumed driving it 20 days ago. Any help or suggestions would be greatly appreciated as some doorknob damaged my '05 Pontiac grand Am for $100 more than I.C.B.C. Black book value