1994 Oldsmobile Ciera Goes out of OD/4th gear

Two symptoms: 1) About 3 weeks ago it started to act up. When going up a slight grade the engine would speed-up and slow-down at about 1 second interval. I think that it was shifting from 4th gear to 3rd gear, back and forth. 2) Yesterday on the freeway on a long trip it seemed like the engine was running fast. I dropped the transmission from OD to D and no change in engine sound. I was consuming more gas than normal. I stopped for gas and tested the OD to D again. This time it worked. Later in the trip the engine was running fast again and the OD to D test failed. I need help in getting this resolved, answered and someone to walk me through the fix. Thanks