1994 Chrysler New Yorker transmission not going into gear

When I start the car in the morning it can take up to a few minutes before it will go into drive. It may go into reverse and still not go into drive. So I put it in drive and just sit until it finally goes into gear. Then while driving to work the transmission seems to want to stay in low. If it does finally shift it will sometimes when slowing down drop out of gear completely and appear to be in neutral. Then I'll have to pull over and wait a few minutes for it to decide to go back into gear.
This has been going on for years and I've just put up with it because the car is so old. Just recently it has got worse. I've had it since 1999 when I bought it from the original owner in perfect condition with only 16000 miles. I only drive 6 miles a day and never take it out on the road. Does this car need to have the transmission rebuilt. Is it a speed sensor or does the fluid need changed. There are no leaks and the fluid level is good.