2001 Jeep Grand Cherokee - Driving 40 mph, motor doesn't cut off but tachometer drops to 0 rpm then goes back up. No engine light

Have been having trouble also with losing power at idle or slowing. No coughing, sputtering or anything. Everything just shuts down. It will start right up after. Have been driving with the overdrive off…the other day, the tach dropped to zero, car did not shut off, but it wouldn’t accelerate. It caught and we took off, but it had gone back into overdrive and I didn’t do it. Then this morning, I was driving down the highway at about 40 mph, tach went to zero, then it caught and off we went. Any ideas?