You have a warped head. The bottom surface may have been cut straight, but the top can still be warped. I have never seen a cam break in a Mercedes. The other thing is to check the amount of oil getting to the shaft.
You can take out the rockers and install the cam without the bolts in the bearing caps and just look to see how things are. This will give and idea of the condition of the straightness of the head.
1992 500E (very soon I hope
1981 280GE SWB
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