i tell you what the cure is don't use brass keys!
if you look at the picture it is clear that the key is brass that is a major reason for them to fail, everyone should avoid brass keys at all costs!! i work at a mercedes shop, and 99% of jammed ignition locks the customers are using brass keys, usually if they are brass the have been copied by someone other that mercedes, go to your dealer spend a few dollars and get a steel key, all they need to order the key is the vin number and proof of ownership. the brass ones loose shavings over time and this will jam the tumblers not allowing them to turn, especially on higher mileage cars, the steel ones do not loose shaving and usually last much longer