What would cause my A604 (97,000 miles) transmission to go into limp mode & later operate properly. This has happened 3 times to me. It went into limp mode so I pulled over & checked fluid, it was full so I put in gear & it operated properly for a few miles then it did it again. I was concerned & did not want to harm the tranny so I had the car towed home. After the car was off the hook I drove the car around the block & it shifted properly again. What could it be?