I have a 2010 Outback with 145,000 miles. In October the throw-out bearing failed requiring a clutch repair. a week following the repair the transmission would pop out of reverse. My mechanic determined that a replacement transmission was needed. Weighing the cost options (new, used, rebuild) I went with a used transmission (with a warrantee). The replacement transmission worked for a week then developed the same problem. I am on my 2nd replacement transmission and now after 2 months it pops out of reverse. What is causing this or is being overlooked here?