Vent Axia window fan slows down?

I have a 9" Vent Axia fan that after about 20mins of operation, slows down. The hub of the fan gets very hot and I'm guessing this is possibly the problem because after I've switch it off to cool down and switch it on again, the fan operates at full strength until it gets hot again.

Is this repairable and how do I go about it? I'm not sure if there is a thermostat somewhere attached to the fan, if so, maybe someone can tell me where!

Thanks in advance for your replies
Joe