The most common problem is that overtime the motor brushes, leave carbon on the commutator. This causes the power transistors to burn out.
Either replace the motors or clean them, and then replace the power transistors. On ebay they sell spare boards but if you have not repaired the motor , you will just burn out the transistors again.
I am probably going to buy the Fanatec CSL DD hub (costs about $599) and somehow get it to work with the Logitech wheel.