I would be very surprised if both stuck and unstuck simultaneously. There is, however, a TSB regarding intermittent operation of speedo and/or tach. The cause is listed as "Vehicle speed signal or RPM signal affected by dampness or corrosion inside kickdown cutout relay module due to water entry"
This relay is listed as 'N15' on all electrical diagrams, but darned if I can find a drawing to show me exactly where it is.

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'96 C280 (gone)
'97 C36
'05 C230k