Pelican Parts Forums - Faulty Speedometer Sensor Saga

I have struggled for sometime to try to identify why the speedometer on my 87 911 does not operate properly. I originally sent my speedo out to North Hollywood Speedometer and Clock. They returned it reporting it was in good operating condition. I reinstalled it but it would still not operate.
I next changed the sensor in the tranny using a generic brand sensor I purchased from our host. The speedometer would now work, but only partially, and with erratic readings - at 70 mph, it would only register 35-40mph or show nothing for slower speeds.
After reading a similar post by a fellow forum member (mmbowers@carolina.rr.com), he suggested that the replacement generic sensor may be the problem. (When fully seated in the tranny, the reed switch in the generic sensor would not close when the magnets inside the tranny were rotated. However when the sensor was partially pulled out, and the magnets rotated,it would work fine. All well and good but I can't drive the car that way.
I next ordered an OEM Porsche sensor from our host. I installed it today and PROBLEM SOLVED - the speedometer works perfectly. Clearly, the money i 'saved' buying the generic sensor was not worth it.