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Will be going 50, speedo goes to 0 then back to 50 or whatever speed you are going. MPH is accurate. Does it a couple times a day. Any suggestions for troubleshooting ?

 
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Grab a scantool. Is there a P0500 series code? (P0500, 501, 502) If there is a code like these, then the PCM is losing the speed sensor signal too, and you need to figure out why (sensor or wiring?)

Depending on how many items are affected, there are a few places wiring could be an issue. Or other components that use the same circuit could be causing the problem. PCM, Cruise, Carlink (if equipped) and the gauge cluster all have to share the same signal.

Drive with the scantool attached and reading the PCM data list. See what the reading for VSS does when the speedo drops.

Got cruise? Use it. See if it acts screwy when the speedo drops off.

 
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So the PCM was replaced about a month agao but it was misbehaving prior to that. I should check the error codes and see if anything is left behind,,,

 
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That's the place to start, and it is easy.
Second is to drive while watching datalist, see what the PCM sees when it drops.

The PCM may not set any code if there is a signal that is plausible, so you want to see what is going on.

Then evaluate the other stuff.

I try to do as much diagnosis as I can from the comfort of the drivers seat, before getting my fingers dirty. Gathering relevant info is part of diagnosis.

 
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Looks like the PCM would have to completely lose the VSS signal for AT LEAST 5 seconds before it can set a P0500 code.

So yes, I would definitely be reading PCM data to see what is happening during an event.