Problems with transmission shifting?

1992 Oldsmobile Delta 88 mileage: 172,000. I have begun to have problems with the transmission (automatic 4 speed) on this car in the last five thousand miles. When the car is cold or has sat for over an hour it does not go into gear. I shift the gear lever up through the range of gears and after several minutes, it will engage. Once it is engaged, the car runs fine and shifts throughout its normal range. However several times my wife has come to an intersection to a stop and had the car slip out of gear but then go right back in. I have begun to think it is either a problem with the linkage or the vacuum modulator. Also, the speedometer cable has begun to stick, that is it jumps from 0 mph to 15 mph to 30 mph etc. And does not move smoothly.