shifts back and forth in DRIVE going up hill
2001 mercury sable - 51K miles
This morning, had a bad THUNK when going from drive to reverse. Later, I noticed that it drove fine on the highway in over-drive, but then going uphill at about 35MPH, there was what appeared to be rapid shifting when in drive. Never had this and don't know what it is. Shifted back to overdrive and it goes away.
Appreciate any ideas about what this might be.