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Code 23 happens if you didn't hold pedal to the floor while the self test was running.

Do you have the engine speed signal and vehicle speed signals connected to the PCM? It needs both of those.
 

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Self test is performed when you read the codes.
 

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I hate replacing parts without a good diagnosis, but sometimes it's the only way. Replace the MLPS (Manual Lever Position Sensor,) sometimes known as the TRS (Transmission Range Sensor.) It is the sensor on the driver's side of the trans with the wiring harness attached.

When this sensor begins to fail it indicates that you have moved the shift lever to N when in reality it hasn't moved. The PCM doesn't know this, all it has is what the sensor tells it. So it obediently sets the trans to neutral. This seems to fit what you're seeing.
 

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The resistance on the MLPS will be fine.

The speed signal could very well be your issue. If it drops enough it will cause the trans to downshift, which will feel like nuetral.
 
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