"By the book" a 930 or 911 should not require valve adjustment more frequently than every 15K miles. Track cars get adjusted a bit more often - normally every 5 - 10K miles.
If you have valves loosening up significantly before that, I would suggest the following:
1. Redo the valve adjustment, making sure that the feeler gauge is well oiled before adjusting each valve. Make sure that the jam nuts are not and have not previously been overtightened. If they have, you should replace the jam nuts (they are about $1.00 each).
2. Make sure that your rocker shafts are not "walking" in their bores. A walking rocker can seriously throw off a valve adjustment. If they are, put them back in their correct position and tighten them down, then readjust the valves.
Es geht nicht darum wie schnell man faehrt, sondern wie gut man schnell fahren kann.
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