It's probably worn brushes. When brush length nears the end of the holder, the spring compression against the armature lessens to the point that good electrical contact is lost. Motor usually becomes intermittent and finally, the brushes will tumble and motor won't run at all.

Cleaning the residual carbon brush material is not likely to improve the blower motor longetivity, since brush length wear is the problem. Unfortunately the brushes are not designed to be replaceable. If you are extremely determined, you might be able to remove and replace them w/ aftermarket brushes. However, this is not likely to be a long term fix due to wear in the blower bearings which is part of reason for brush wear. Given the effort to access the blower, I would go w/ aftermarket or OEM blower. Tough call given the cost differential. Yes, the OEM is a better product. I've had good luck w/ aftermarket, but not perfect. Used seems less attractive unless you can confirm the mileage on the donor car was low.

good luck