I agree with whunter. If you don't feel comfortable working on it yourself take it in to get looked at now. If it seizes it gets very expensive.

I would seriously think about removing the tumbler, its easy to do, and check to see if its binding in any way at all. I would do that before I even thought about ordering a new one. Lubricating the tumbler with a graphite powder is a good idea. But I don't think it will help your situation. It really sounds like you have a failure in the internal mechanism of the assembly.

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'84 300CD Turbo 132k (Anthracite Grey) - WVO - My daily driver - Recently named coo-coo-coupe by my daughter.
'84 300D Turbo 240k (Anthracite Grey) - Garage Queen
'83 300D Turbo 220k (Orient Red) - WVO - Wifes daily driver

I'm not a certified mechanic, but I did stay at a HolidayInn Express last night.