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Sounds like you have a bad output speed sensor. A scan tool that can read live data on the transmission would verify this. Replace the output speed sensor and your problem should go away.
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You need a scan tool to see live data of the trans, then you can see output speed in real time. That would rule out the wiring harness. You could also have a mechanical problem in the trans itself.
You have an intermittent problem in the wiring harness.