|First, your original speedometer board was without a doubt bad. It had an open trace on the main signal line so there's no way it could have worked.
Trying to fix the problem, you replaced the speed sensor. More than likely, the only real problem was that the speedometer board was bad. The only thing that changed in the original configuration when the speedometer used to work was changing the speed sensor.
I started searching the web and found a few reports where people have changed their speed sensor with one other than OEM and they had the same sort of problems the you are seeing. I see a lot of non-OEM sensors available for the Z. There are lots of them on Ebay. These are junk parts from China or Tiawan.
You said you bought the speed sensor from COZ. I wouldn't put it past COZ to sell cheap chinese junk as OEM because of the things he's done in the past. I would definitely start with the speed sensor since that has changed from the way everything was before.
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