Transmission jerking

[wonbill]

Hi Guys/Gals

I have recently developed a transmission problem (I think)
My 1999 Grand Masquis (about 100000 miles on it)is driven about 2-3 months a year . This year when it was put on the road; on a hill it seemed to change from overdrive to drive back and forth 2-3 times before decided which gear it wanted to go into. now it seems to go back and forth between 2nd and drive if I have the cruise and overdrive turned off. if I give it gas when the back and forth starts this seems to make it's decision faster.
Any thoughts or reccomendations to tell the transmission shop on what to look for?
Thanks one and all for taking the time to read this
Wonbill