The speedometer is part of the instrument cluster. Did you have the speed sensor changed?
How often does this happen?
Have they been able to duplicate the problem when testing?
Does the weather make a difference?
Was your replacement cluster used or new?
Out of curiosity are you using Castriota Chevy on West Liberty?
Did someone hook up a tech2 immediately and take some readings? They should have done a gauge sweep test and then checked Vehicle Speed Sensor operation while it was in a failure state.
For proper diagnosis, someone will need to perform a gauge sweep test and also verify the Vehicle Speed Sensor input when the fault is occuring. Without a factory scan tool I can't diagnose any further.
You need to find out the results of the above tests.
There could be a failure in the cruise control module or the PCM which is causing this problem.
You could try disconnecting the cruise control module connector to see if it is shorting out the system. It is located on the passenger side of the engine compartment: