transmisson?

I have a 1997 Ford Contour with about 130,000 miles on it which has started shifting in and out of passing gear without acceleration. For example going up hill it will shift into and out of gear several times. This has only recently started and overall this car has had no problems until now. I want to donate this to a needy neighbor but want to solve the problem if I can. The transmission was serviced about 20,000 miles ago if that helps.

Any ideas and help would be greatly appreciated. Thank you.