Caframo fan 802 reliability

In the last three years I bought or replaced five of these fans. I install them at the back of the stove as recommended but after several months they start to slow down or/and just won't start. Caframo will repair or replace them but still have the same problems after few months of use. Not sure if it's the motor or something else that goes wrong but they sure can use some reliability improvements.
Warranty should be more than one year under these circumstances.