Check for fault codes first, but the problem most likely is the very common sticking turbo vanes causing an overboost, which is even more likely on an automatic due to the more gentle way the car is likely to be driven. Limp home mode itself is not a fault, but a safety protection against engine damage which could otherwise occur if the turbo boost pressure was uncontrolled.

Innotec Turbo Clean is a product specially designed for cleaning the turbo vanes. I've used it several times and it's worked every time for me. It's worth a try, it's much cheaper than a new turbo.