Your gears is stripped and you need a new one or that one rebuilt!!
could be failing transmission
Your transmission might automatically shift out of overdrive while driving on the interstate because of low transmission fluid. This issue might also occur because of a bad transmission sensor or overdrive sensor.
It might be a broken motor mount or transmission mount.
Sump compression
A transmission is going bad when you begin to notice the vehicle driving or shifting unusually. A diagnosis by a competent technician would be recommended as some symptoms that feel like transmission concerns may actually be engine related.
The overdrive warning light might start flickering when you are driving because of low transmission fluid. This light could also come on because of tension on the transmission while you are towing a boat or trailer.
On the vast majority of modern cars today it is impossible to actually shift to reverse while driving at high speed. Modern transmissions have an override safety device that prevents the transmission from going into reverse at any speed forward that would harm the transmission. You could put the selector in reverse, but the transmission would not shift to reverse. On older transmissions without this feature, you would probably destroy the transmission and possible have an accident.
if you get dirt in a transmission while drive 1000 miles you could ruin it. no question about it...
Check transmission fluid level Change transmission fluid and filter (if it has one)
That would be your transmission. It may be slipping, or you may be lossing a gear. Have it checked.
No, but why would you?
worn out trans most likely clutchs or oneway clutch <sprag>