I had the same problem. I tried cleaning the look, lubricating, etc, etc. I ended up changing the lock with a new one from MB. It took about 1-2 weeks to arrive. It comes with a new key (will fit the rest of your locks) and the lock for around $200. I figured it was either that, or buying an aftermarket keyless entry alarm for more than $200. I decided to keep it stock.
No more problems...
[This message has been edited by J.HIDALGO (edited 11-03-2000).]