First day I took mine to work, I got in it at the end of a (hot Texas) day, and had this problem. I called the PO and he said "you just have to jiggle it." II did and it finally turned - I had read all about this problem on these forums, so I drove straight to the nearest parts store (Autozone) and got a tube of graphite powder. Squirted it in till I saw a puff and it is much better now (not fixed, just not so scared it is going to lock forever). Did this everytime it stuck and over the next week, it happened less and less often -

Definitely just buying me some time, I have a new key core on my short wish list. But it bought me some peace of mind for about $4

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Current Mercedes
1979 maple yellow 240D 4-speed

Gone and fondly remembered:
1980 orient red 240D 4-speed

Gone and NOT fondly remembered:
1982 Chna Blue 300TD

Other car in the stable:
2013 VW Jetta Sportwagen TDI / 6-speed MT